Add policyengine-uk validation and fix SCP parameters

Author: MaxGhenis

.github/workflows/validate.yml

@@ -0,0 +1,43 @@
+name: Validate
+
+on:
+  pull_request:
+    paths:
+      - "src/**"
+      - "parameters/**"
+      - "scripts/**"
+      - "Cargo.toml"
+      - "Cargo.lock"
+      - ".github/workflows/validate.yml"
+  push:
+    paths:
+      - "src/**"
+      - "parameters/**"
+      - "scripts/**"
+      - "Cargo.toml"
+      - "Cargo.lock"
+      - ".github/workflows/validate.yml"
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: dtolnay/rust-toolchain@stable
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.13"
+
+      - name: Run Rust tests
+        run: cargo test
+
+      - name: Build Rust CLI
+        run: cargo build
+
+      - name: Install PolicyEngine UK reference model
+        run: python -m pip install --upgrade pip policyengine-uk==2.72.3
+
+      - name: Validate against PolicyEngine UK
+        run: python scripts/validate_against_policyengine_uk.py --rust-binary target/debug/policyengine-uk-rust

LEGISLATIVE_REFERENCE.md

@@ -719,14 +719,15 @@ source, and — for projected years — the methodology used to derive them.
 SIs: SI 2023/233 and SI 2023/234 (2023/24); SI 2024/242 (2024/25); SI 2025/295 and SI 2025/292
 (2025/26).
 
-**Projected years (2026/27 – 2029/30)**: derived from the OBR Economic and Fiscal Outlook March
-2026 growth factor forecasts. Benefits uprated by September CPI; state pension by the triple
-lock (max of CPI, earnings, 2.5%); income tax thresholds frozen until 2027/28 then CPI-uprated.
-
-**Known issues in confirmed years** (to be fixed separately):
-- 2023/24 NI `main_rate` 0.115 is blended — the underlying rates are 12% and 10% (mid-year cut)
-- 2025/26 Scottish Child Payment `weekly_amount` should be £27.15 (SSI 2025/100), not £26.70
-- 2026/27 onward: Scottish Child Payment not uprated (still showing £26.70 — needs correction)
+**Projected years (2026/27 – 2029/30)**: generally derived from the OBR Economic and Fiscal
+Outlook March 2026 growth factor forecasts. Benefits uprated by September CPI; state pension by
+the triple lock (max of CPI, earnings, 2.5%); income tax thresholds frozen until 2027/28 then
+CPI-uprated. Scottish Child Payment follows the Scottish Fiscal Commission January 2026 forecast
+path already used in `policyengine-uk`.
+
+**Known modelling approximations**:
+- 2023/24 NI `main_rate` 0.115 is represented as a fiscal-year blended annual rate to reflect the
+  January 2024 mid-year cut from 12% to 10%
 
 ---
 

parameters/2025_26.yaml

@@ -154,8 +154,8 @@ tax_credits:
 
 scottish_child_payment:
   # Social Security (Scotland) Act 2018; SSI 2020/351
-  # £26.70/week from November 2024
-  weekly_amount: 26.70
+  # £27.15/week from April 2025
+  weekly_amount: 27.15
   max_age: 16.0
 
 uc_migration:

parameters/2026_27.yaml

@@ -104,7 +104,7 @@ tax_credits:
 
 
 scottish_child_payment:
-  weekly_amount: 26.70
+  weekly_amount: 28.20
   max_age: 16.0
 
 uc_migration:

parameters/2027_28.yaml

@@ -102,7 +102,7 @@ tax_credits:
 
 
 scottish_child_payment:
-  weekly_amount: 26.70
+  weekly_amount: 28.85
   max_age: 16.0
 
 uc_migration:

parameters/2028_29.yaml

@@ -103,7 +103,7 @@ tax_credits:
 
 
 scottish_child_payment:
-  weekly_amount: 26.70
+  weekly_amount: 29.45
   max_age: 16.0
 
 uc_migration:

parameters/2029_30.yaml

@@ -102,7 +102,7 @@ tax_credits:
 
 
 scottish_child_payment:
-  weekly_amount: 26.70
+  weekly_amount: 30.05
   max_age: 16.0
 
 uc_migration: