Add Scottish Child Payment 2026-27 rate and CPI uprating (#1487)

* Add Scottish Child Payment 2026-27 rate and CPI uprating

- Add SCP 2026-27 rate of £28.20/week (effective 2026-04-01) per
  Scottish Fiscal Commission forecasts
- Enable CPI uprating for SCP base amount parameter
- Enable CPI uprating for SCP baby boost under-1 total parameter
- Add reference to Scottish Fiscal Commission January 2026 forecasts
- Add unit test for 2026 rate

Reference: Scottish Fiscal Commission - Scotland's Economic and Fiscal
Forecasts January 2026
https://fiscalcommission.scot/wp-content/uploads/2026/01/Scotlands-Economic-and-Fiscal-Forecasts-January-2026-revised-13-01-2026.pdf

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Add SFC forecast values for 2027-2031

Update parameters with explicit Scottish Fiscal Commission forecast values:

Scottish Child Payment base rate:
- 2026-27: £28.20
- 2027-28: £28.85
- 2028-29: £29.45
- 2029-30: £30.05
- 2030-31: £30.65

Under-1 total:
- 2027-28: £40.00
- 2028-29: £40.80
- 2029-30: £41.65
- 2030-31: £42.50

Update tests to match SFC forecast values.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Fix changelog entry format

Move changelog entry from changelog.yaml to changelog_entry.yaml per
project conventions.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* style: apply Black 26.1.0 formatting

Remove extra blank lines per updated Black style guide.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: vahid-ahmadi

changelog_entry.yaml

@@ -0,0 +1,6 @@
+- bump: minor
+  changes:
+    added:
+      - Scottish Child Payment rates from 2026-27 to 2030-31 per Scottish Fiscal Commission forecasts.
+      - Scottish Child Payment baby boost under-1 total from 2027-28 to 2030-31 per SFC forecasts.
+      - CPI uprating for SCP parameters for projections beyond 2030-31.

policyengine_uk/parameters/gov/contrib/scotland/scottish_child_payment/under_one.yaml

@@ -1,11 +1,17 @@
 description: Total weekly Scottish Child Payment amount for children under 1.
 values:
   0001-01-01: 0
-  2027-04-01: 40
+  2027-04-01: 40.00
+  2028-04-01: 40.80
+  2029-04-01: 41.65
+  2030-04-01: 42.50
 metadata:
   unit: currency-GBP
   period: week
   label: Scottish Child Payment total for under-1s
+  uprating: gov.economic_assumptions.indices.obr.consumer_price_index
   reference:
     - title: Scottish Budget 2026 to 2027
       href: https://www.gov.scot/publications/scottish-budget-2026-2027/pages/6/
+    - title: Scottish Fiscal Commission - Scotland's Economic and Fiscal Forecasts January 2026
+      href: https://fiscalcommission.scot/wp-content/uploads/2026/01/Scotlands-Economic-and-Fiscal-Forecasts-January-2026-revised-13-01-2026.pdf

policyengine_uk/parameters/gov/hmrc/fuel_duty/calculate_fuel_duty_rates.py

@@ -18,7 +18,6 @@
 
 import yaml
 
-
 # Years to calculate rates for
 CALCULATION_YEARS = range(2026, 2031)
 

policyengine_uk/parameters/gov/social_security_scotland/scottish_child_payment/amount.yaml

@@ -6,12 +6,20 @@ values:
   2022-11-14: 25
   2024-04-01: 26.70
   2025-04-01: 27.15
+  2026-04-01: 28.20
+  2027-04-01: 28.85
+  2028-04-01: 29.45
+  2029-04-01: 30.05
+  2030-04-01: 30.65
 metadata:
   unit: currency-GBP
   period: week
   label: Scottish Child Payment weekly amount
+  uprating: gov.economic_assumptions.indices.obr.consumer_price_index
   reference:
     - title: The Scottish Child Payment Regulations 2020, Regulation 20
       href: https://www.legislation.gov.uk/ssi/2020/351/regulation/20
     - title: Scottish Government - Scottish Child Payment
       href: https://www.gov.scot/policies/social-security/scottish-child-payment/
+    - title: Scottish Fiscal Commission - Scotland's Economic and Fiscal Forecasts January 2026
+      href: https://fiscalcommission.scot/wp-content/uploads/2026/01/Scotlands-Economic-and-Fiscal-Forecasts-January-2026-revised-13-01-2026.pdf

policyengine_uk/tests/microsimulation/test_reform_impacts.py

@@ -8,7 +8,6 @@
 from pathlib import Path
 from policyengine_uk import Microsimulation
 
-
 # Load configuration from YAML file
 config_path = Path(__file__).parent / "reforms_config.yaml"
 with open(config_path, "r") as f:

policyengine_uk/tests/microsimulation/test_salary_sacrifice_cap_reform.py

@@ -20,7 +20,6 @@
 import pandas as pd
 from policyengine_uk import Microsimulation
 
-
 # Policy year when the salary sacrifice cap takes effect
 POLICY_YEAR = 2030  # Use 2030 to ensure cap is active (cap starts 2029-04-06)
 

policyengine_uk/tests/policy/baseline/gov/social_security_scotland/scottish_child_payment.yaml

@@ -255,6 +255,72 @@
     # [parent, child_1]
     scottish_child_payment: [0, 1412]
 
+- name: SCP with 2026 amount (£28.20/week)
+  period: 2026
+  absolute_error_margin: 10
+  input:
+    people:
+      parent:
+        age: 30
+      child_1:
+        age: 5
+    benunits:
+      benunit:
+        members: [parent, child_1]
+        universal_credit: 5000
+    households:
+      household:
+        members: [parent, child_1]
+        region: SCOTLAND
+  output:
+    # £28.20/week * 52 weeks = £1,466.40
+    # [parent, child_1]
+    scottish_child_payment: [0, 1466]
+
+- name: SCP with 2027 amount (£28.85/week)
+  period: 2027
+  absolute_error_margin: 10
+  input:
+    people:
+      parent:
+        age: 30
+      child_1:
+        age: 5
+    benunits:
+      benunit:
+        members: [parent, child_1]
+        universal_credit: 5000
+    households:
+      household:
+        members: [parent, child_1]
+        region: SCOTLAND
+  output:
+    # £28.85/week * 52 weeks = £1,500.20
+    # [parent, child_1]
+    scottish_child_payment: [0, 1500]
+
+- name: SCP with 2028 amount (£29.45/week)
+  period: 2028
+  absolute_error_margin: 10
+  input:
+    people:
+      parent:
+        age: 30
+      child_1:
+        age: 5
+    benunits:
+      benunit:
+        members: [parent, child_1]
+        universal_credit: 5000
+    households:
+      household:
+        members: [parent, child_1]
+        region: SCOTLAND
+  output:
+    # £29.45/week * 52 weeks = £1,531.40
+    # [parent, child_1]
+    scottish_child_payment: [0, 1531]
+
 - name: CTC no longer qualifies from 2025 (after April 2024)
   period: 2025
   absolute_error_margin: 1

policyengine_uk/tests/policy/reforms/scp_baby_bonus/scottish_child_payment_baby_bonus.yaml

@@ -24,10 +24,10 @@
         members: [parent, baby]
         region: SCOTLAND
   output:
-    # Under-1s get £40/week total (policy parameter)
-    # £40/week * 52 weeks = £2,080 per year
+    # Under-1s get £40.80/week total (2028-29 SFC forecast)
+    # £40.80/week * 52 weeks = £2,121.60 per year
     # [parent, baby]
-    scottish_child_payment: [0, 2080]
+    scottish_child_payment: [0, 2122]
 
 - name: SCP baby bonus - child over 1 gets standard rate (2028)
   period: 2028
@@ -49,9 +49,9 @@
         members: [parent, toddler]
         region: SCOTLAND
   output:
-    # Standard rate only: £27.15/week * 52 = £1,411.80
+    # Standard rate only: £29.45/week * 52 = £1,531.40
     # [parent, toddler]
-    scottish_child_payment: [0, 1412]
+    scottish_child_payment: [0, 1531]
 
 - name: SCP baby bonus - mixed ages (2028)
   period: 2028
@@ -75,10 +75,10 @@
         members: [parent, baby, older_child]
         region: SCOTLAND
   output:
-    # Baby (age 0): £40/week total * 52 = £2,080
-    # Older child (age 5): £27.15/week * 52 = £1,411.80
+    # Baby (age 0): £40.80/week total * 52 = £2,121.60
+    # Older child (age 5): £29.45/week * 52 = £1,531.40
     # [parent, baby, older_child]
-    scottish_child_payment: [0, 2080, 1412]
+    scottish_child_payment: [0, 2122, 1531]
 
 - name: SCP baby bonus - not in Scotland (2028)
   period: 2028

policyengine_uk/tests/test_behavioral_responses.py

@@ -16,7 +16,6 @@
 from policyengine_uk import Microsimulation
 from policyengine_uk.model_api import Scenario
 
-
 # Check if HF token is available for data-dependent tests
 HF_TOKEN_AVAILABLE = bool(os.environ.get("HUGGING_FACE_TOKEN"))
 requires_hf_data = pytest.mark.skipif(

policyengine_uk/utils/dependencies.py

@@ -179,14 +179,12 @@ def create_waterfall_change_chart(
 
 
 def add_fonts():
-    fonts = HTML(
-        """
+    fonts = HTML("""
     <link rel="preconnect" href="https://fonts.googleapis.com">
     <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
     <link href="https://fonts.googleapis.com/css2?family=Roboto+Serif:ital,opsz,wght@0,8..144,100..900;1,8..144,100..900&display=swap" rel="stylesheet">
     <link href="https://fonts.googleapis.com/css2?family=Roboto+Mono:ital,wght@0,100..700;1,100..700&display=swap" rel="stylesheet">
-    """
-    )
+    """)
     return display_html(fonts)