Revert SCP rate increase and refactor baby bonus to £40 total (#1484)

* Revert "feat: add Scottish Child Payment 2026-27 rate increase (#1483)"

This reverts commit dac23c7.

* Revert SCP rate increase and refactor baby bonus to use £40 total

- Revert PR #1483 (SCP 2026-27 rate increase not yet law)
- Replace baby_bonus.yaml with under_one.yaml (£40/week total parameter)
- Reform now uses max(under_one, base_rate) for under-1s

The Scottish Budget says "£40 a week total" not a fixed bonus amount.

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

---------

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

Author: MaxGhenis

changelog_entry.yaml

@@ -0,0 +1,5 @@
+- bump: patch
+  changes:
+    fixed:
+      - Reverted premature SCP 2026-27 rate increase (not yet law).
+      - Refactored SCP baby bonus reform to use £40/week total as the policy parameter instead of a fixed bonus amount.

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

@@ -0,0 +1,11 @@
+description: Total weekly Scottish Child Payment amount for children under 1.
+values:
+  0001-01-01: 0
+  2027-04-01: 40
+metadata:
+  unit: currency-GBP
+  period: week
+  label: Scottish Child Payment total for under-1s
+  reference:
+    - title: Scottish Budget 2026 to 2027
+      href: https://www.gov.scot/publications/scottish-budget-2026-2027/pages/6/

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

@@ -6,7 +6,6 @@ values:
   2022-11-14: 25
   2024-04-01: 26.70
   2025-04-01: 27.15
-  2026-04-01: 28.20
 metadata:
   unit: currency-GBP
   period: week
@@ -16,5 +15,3 @@ metadata:
       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 Budget 2026-27
-      href: https://www.gov.scot/news/a-budget-to-tackle-child-poverty/

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

@@ -6,20 +6,21 @@ def create_scottish_child_payment_baby_bonus_reform() -> Reform:
     """
     Reform that implements SCP baby bonus for children under 1.
 
-    Policy: Children under 1 receive an additional weekly bonus on top of
-    the standard SCP rate. The bonus amount is parameterized at
-    gov.contrib.scotland.scottish_child_payment.baby_bonus.
+    Policy: Children under 1 receive a total of £40/week (the policy parameter).
+    The "bonus" is implicitly the difference between this total and the base rate.
 
     Source: Scottish Budget 2026-27
-    https://www.gov.scot/publications/scottish-budget-2026-2027-finance-secretarys-statement-13-january-2026-2/
+    https://www.gov.scot/publications/scottish-budget-2026-2027/pages/6/
+    "This will bring the total Scottish Child Payment amount to £40 a week
+    for children under 1."
     """
 
     class scottish_child_payment(Variable):
         label = "Scottish Child Payment"
         documentation = (
             "Scottish Child Payment amount for this child. "
             "Paid to eligible children in families receiving qualifying benefits. "
-            "When baby bonus reform is active, children under 1 receive additional payment."
+            "When baby bonus reform is active, children under 1 receive £40/week total."
         )
         entity = Person
         definition_period = YEAR
@@ -32,27 +33,33 @@ class scottish_child_payment(Variable):
         ]
 
         def formula(person, period, parameters):
-            # Get SCP parameters
+            # Get base SCP rate
             p = parameters(
                 period
             ).gov.social_security_scotland.scottish_child_payment
-            weekly_amount = p.amount
+            base_weekly = p.amount
 
-            # Get baby bonus parameter (age-bracketed)
-            baby_bonus_params = parameters(
+            # Get reform parameters
+            scp_reform = parameters(
                 period
-            ).gov.contrib.scotland.scottish_child_payment.baby_bonus
-            age = person("age", period)
-            baby_bonus = baby_bonus_params.calc(age)
+            ).gov.contrib.scotland.scottish_child_payment
+            in_effect = scp_reform.in_effect
+            under_one = scp_reform.under_one
 
-            # Total weekly amount = base + baby bonus
-            total_weekly = weekly_amount + baby_bonus
+            # For under-1s when reform in effect: use under_one total
+            # Otherwise: use base rate
+            age = person("age", period)
+            weekly_amount = where(
+                (age < 1) & in_effect,
+                under_one,
+                base_weekly,
+            )
 
             # Child-level take-up (generated stochastically in dataset)
             would_claim = person("would_claim_scp", period)
 
             # Convert to annual amount
-            return total_weekly * WEEKS_IN_YEAR * would_claim
+            return weekly_amount * WEEKS_IN_YEAR * would_claim
 
     class reform(Reform):
         def apply(self):

policyengine_uk/reforms/scotland/scottish_child_payment_reform.py

@@ -415,27 +415,3 @@
     # £27.15/week * 52 weeks = £1,411.80
     # [grandparent, child_1]
     scottish_child_payment: [0, 1412]
-
-# 2026-27 rate increase
-
-- 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]

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

@@ -1,8 +1,8 @@
 # Scottish Child Payment Baby Bonus Reform tests
-# Tests that the SCP baby bonus reform correctly applies additional
-# payment for children under 1 year old.
+# Tests that the SCP baby bonus reform correctly applies the £40/week
+# total for children under 1 year old.
 # Policy effective from 2027-28 fiscal year (April 2027).
-# Reform adds £12.85/week bonus for under-1s on top of standard rate.
+# Policy parameter: £40/week total for under-1s (not base + bonus).
 
 - name: SCP baby bonus for child under 1 (2028)
   period: 2028
@@ -24,7 +24,7 @@
         members: [parent, baby]
         region: SCOTLAND
   output:
-    # Base: £27.15/week + Bonus: £12.85/week = £40/week total
+    # Under-1s get £40/week total (policy parameter)
     # £40/week * 52 weeks = £2,080 per year
     # [parent, baby]
     scottish_child_payment: [0, 2080]
@@ -75,7 +75,7 @@
         members: [parent, baby, older_child]
         region: SCOTLAND
   output:
-    # Baby (age 0): £40/week * 52 = £2,080
+    # Baby (age 0): £40/week total * 52 = £2,080
     # Older child (age 5): £27.15/week * 52 = £1,411.80
     # [parent, baby, older_child]
     scottish_child_payment: [0, 2080, 1412]
@@ -124,7 +124,7 @@
         members: [parent, baby]
         region: SCOTLAND
   output:
-    # Before 2027-04-01 - baby bonus is £0, so standard rate only
+    # Before 2027-04-01 - under_one_total is £0, so standard rate applies
     # £27.15/week * 52 = £1,411.80
     # [parent, baby]
     scottish_child_payment: [0, 1412]