Stop paying Tax Credits after DWP managed migration completed (#1619)

* Zero out Tax Credits after DWP managed migration completed

Working Tax Credit and Child Tax Credit were still paying ~£1.9bn in
2025 baseline simulations even though DWP's managed migration to
Universal Credit completed on 5 April 2024 and no tax credit awards
have been paid since.

The existing `tax_credits` formula had no time-based gate; it paid out
whenever `working_tax_credit_reported` or `child_tax_credit_reported`
was non-zero in the input data (which the FRS 2023-24 build
understandably still carries).

Adds a `gov.dwp.tax_credits.active` bool parameter that flips to
`false` on 2024-04-06 and uses it to short-circuit the `tax_credits`
formula. Post-fix aggregates (enhanced FRS 2023-24, 1.50.3):

| year | before | after |
| ---  | ---:   | ---:  |
| 2024 | £1.88bn | £0.00bn |
| 2025 | £1.86bn | £0.00bn |
| 2026 | £2.11bn | £0.00bn |

UC aggregates are unchanged (those claimants were already on UC in the
enhanced FRS calibration), so this removes £1.9bn of phantom baseline
spending per year.

Adds two YAML regression tests covering the post-migration zero case
and a future-year check. Existing tax_credits.yaml tests all pass
(they use period 2020, so active=true).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Correct Tax Credits end date: 5 April 2025, not 5 April 2024

DWP/HMRC's official page (https://www.gov.uk/tax-credits-have-ended)
confirms Working Tax Credit and Child Tax Credit ended on
5 April 2025, the end of the 2024-25 tax year — not 2024-04-06 as
originally encoded. The earlier date was based on an earlier scheme
closure (to new claims) rather than the final ending of awards.

Moves the `active` flip to 2025-04-06, updates the test suite so:

- 2024 still produces non-zero tax_credits (baseline ~£1.88bn paid,
  £2,607/yr in the synthetic test case)
- 2026 and later produce zero
- The "all future years" regression at 2030 stays in place

Verified aggregates (enhanced FRS 2023-24, 1.50.3):

| year | paid WTC+CTC |
| ---  | ---:         |
| 2023 | £1.76bn (unchanged) |
| 2024 | £1.88bn (preserved) |
| 2025 | £0.00bn (was £1.86bn) |
| 2026+ | £0.00bn |

Reform-reactivation works via the standard parameter-override API,
e.g. `{'gov.dwp.tax_credits.active': {'2026': True}}` restores
tax_credits to about £5.9bn in 2026.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: MaxGhenis

changelog.d/tax-credits-post-migration.fixed.md

@@ -0,0 +1 @@
+- Fix Working Tax Credit and Child Tax Credit continuing to pay out from the 2025-26 tax year onward. Working Tax Credit and Child Tax Credit ended on 5 April 2025 (HMRC/DWP). Adds a `gov.dwp.tax_credits.active` parameter that flips to `false` on 2025-04-06 and gates `tax_credits` on it. Removes about £1.9bn of phantom Tax Credit spending per year from 2025-26 onward while preserving the legitimate 2024-25 baseline.

policyengine_uk/parameters/gov/dwp/tax_credits/active.yaml

@@ -0,0 +1,14 @@
+description: Whether the Tax Credits scheme is actively paying awards. Working Tax Credit and Child Tax Credit ended on 5 April 2025; final finalisation and overpayment recovery may continue, but no ongoing awards are paid after that date.
+values:
+  2003-04-06: true
+  2025-04-06: false
+metadata:
+  label: Tax Credits scheme active
+  unit: bool
+  reference:
+    - title: "HMRC/DWP: Tax credits ended on 5 April 2025"
+      href: https://www.gov.uk/tax-credits-have-ended
+    - title: "House of Commons Library: Completing Universal Credit rollout (CBP-9984)"
+      href: https://commonslibrary.parliament.uk/research-briefings/cbp-9984/
+    - title: Tax Credits Act 2002
+      href: https://www.legislation.gov.uk/ukpga/2002/21/contents

policyengine_uk/tests/policy/baseline/finance/benefit/family/tax_credits_post_migration.yaml

@@ -0,0 +1,38 @@
+- name: Tax credits still paid in 2024 (scheme active until 5 April 2025)
+  period: 2024
+  absolute_error_margin: 1
+  input:
+    age: 35
+    weekly_hours: 40
+    employment_income: 10000
+    working_tax_credit_reported: 3000
+    claims_all_entitled_benefits: true
+  output:
+    tax_credits: 2607
+
+- name: Tax credits are zero after scheme ended on 5 April 2025
+  period: 2026
+  absolute_error_margin: 0
+  input:
+    age: 35
+    weekly_hours: 40
+    employment_income: 10000
+    working_tax_credit_reported: 3000
+    child_tax_credit_reported: 2000
+    claims_all_entitled_benefits: true
+  output:
+    tax_credits: 0
+    working_tax_credit: 0
+    child_tax_credit: 0
+
+- name: Tax credits remain zero in all future years
+  period: 2030
+  absolute_error_margin: 0
+  input:
+    age: 35
+    weekly_hours: 40
+    employment_income: 10000
+    working_tax_credit_reported: 3000
+    claims_all_entitled_benefits: true
+  output:
+    tax_credits: 0

policyengine_uk/variables/gov/dwp/tax_credits.py

@@ -9,9 +9,11 @@ class tax_credits(Variable):
     definition_period = YEAR
     unit = GBP
 
-    def formula(person, period, parameters):
+    def formula(benunit, period, parameters):
+        if not parameters(period).gov.dwp.tax_credits.active:
+            return benunit.empty_array()
         amount = add(
-            person,
+            benunit,
             period,
             ["working_tax_credit_pre_minimum", "child_tax_credit_pre_minimum"],
         )