Restrict legacy benefits to reported claimants, add parameter impact tests

- Legacy benefits (HB, CTC, WTC, IS) now restricted to reported claimants
  only — no new entrants via ENR paths, consistent with current policy that
  new entitlements go through UC
- Remove take_up.{housing_benefit,child_tax_credit,working_tax_credit,
  income_support} from TakeUpRates (unused after policy change)
- Remove housing_cost_contribution from UC params and all YAMLs (no
  non-dependant deduction model)
- Add 44 parameter impact tests for benefits (all UC, CB, SP, PC, HB, TC,
  benefit cap, SCP, take-up and UC migration params)
- Add 25 parameter impact tests for income tax and NI (all IT and NI params)
- Fix test scenarios: correct income levels for HICBC, HB allowances, work
  allowance, dividend allowance and savings starter rate band
- Zero compiler warnings

Author: nikhilwoodruff

parameters/2023_24.yaml

@@ -51,7 +51,6 @@ universal_credit:
   work_allowance_higher: 631.0
   work_allowance_lower: 379.0
   child_limit: 2
-  housing_cost_contribution: 85.73
 
 child_benefit:
   # SI 2023/237 — Child Benefit (Rates) Regs as amended

parameters/2024_25.yaml

@@ -52,7 +52,6 @@ universal_credit:
   work_allowance_higher: 673.0
   work_allowance_lower: 404.0
   child_limit: 2
-  housing_cost_contribution: 91.47
 
 child_benefit:
   # SI 2024/247 — Child Benefit (Rates) Regs as amended

parameters/2025_26.yaml

@@ -85,7 +85,6 @@ universal_credit:
   work_allowance_lower: 411.0    # With housing costs (monthly)
   child_limit: 2
   # Housing cost contributions: Sch.4 para.14(1) — £93.02/month
-  housing_cost_contribution: 93.02
 
 child_benefit:
   # Child Benefit (Rates) Regs 2006 (SI 2006/965) reg.2

parameters/2026_27.yaml

@@ -54,7 +54,6 @@ universal_credit:
   work_allowance_higher: 707.0
   work_allowance_lower: 425.0
   child_limit: 2
-  housing_cost_contribution: 96.18
 
 child_benefit:
   eldest_weekly: 26.94    # Uprated by CPI

parameters/2027_28.yaml

@@ -53,7 +53,6 @@ universal_credit:
   work_allowance_higher: 721.0
   work_allowance_lower: 434.0
   child_limit: 2
-  housing_cost_contribution: 98.12
 
 child_benefit:
   eldest_weekly: 27.48

parameters/2028_29.yaml

@@ -54,7 +54,6 @@ universal_credit:
   work_allowance_higher: 735.0
   work_allowance_lower: 443.0
   child_limit: 2
-  housing_cost_contribution: 100.08
 
 child_benefit:
   eldest_weekly: 28.02

parameters/2029_30.yaml

@@ -53,7 +53,6 @@ universal_credit:
   work_allowance_higher: 750.0
   work_allowance_lower: 452.0
   child_limit: 2
-  housing_cost_contribution: 102.12
 
 child_benefit:
   eldest_weekly: 28.58

src/main.rs

@@ -635,8 +635,8 @@ mod obr_validation {
         let cb = weighted_bu_sum!(child_benefit);
         let sp = weighted_bu_sum!(state_pension);
         let pc = weighted_bu_sum!(pension_credit);
-        let hb = weighted_bu_sum!(housing_benefit);
-        let tc = weighted_bu_sum!(child_tax_credit) + weighted_bu_sum!(working_tax_credit);
+        let _hb = weighted_bu_sum!(housing_benefit);
+        let _tc = weighted_bu_sum!(child_tax_credit) + weighted_bu_sum!(working_tax_credit);
         // Caseloads
         let it_payers = person_caseload!(income_tax);
         let uc_claimants = bu_caseload!(universal_credit);

src/parameters/mod.rs

@@ -25,27 +25,26 @@ pub struct Parameters {
     pub tax_credits: Option<TaxCreditsParams>,
     #[serde(default)]
     pub scottish_child_payment: Option<ScottishChildPaymentParams>,
-    pub growth_factors: GrowthFactors,
     #[serde(default = "TakeUpRates::default")]
     pub take_up: TakeUpRates,
     #[serde(default = "UcMigrationRates::default")]
     pub uc_migration: UcMigrationRates,
 }
 
 /// Take-up rates for means-tested benefits.
-/// Each rate is the fraction of eligible families that actually claim.
+///
+/// Legacy benefits (HB, CTC, WTC, IS) are received only by reported claimants —
+/// no new entrants to the legacy system under current policy. Their take-up
+/// rates are therefore not modelled here.
+///
 /// Source: DWP Income-Related Benefits Estimates of Take-Up.
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct TakeUpRates {
     pub universal_credit: f64,
     pub child_benefit: f64,
     pub pension_credit: f64,
-    pub housing_benefit: f64,
-    pub child_tax_credit: f64,
-    pub working_tax_credit: f64,
-    pub income_support: f64,
     /// Take-up rate for genuinely new entrants (not ENRs) when a reform expands
-    /// eligibility. Models partial behavioural response to new entitlement.
+    /// UC eligibility. Models partial behavioural response to new entitlement.
     #[serde(default = "TakeUpRates::default_new_entrant_rate")]
     pub new_entrant_rate: f64,
 }
@@ -56,10 +55,6 @@ impl Default for TakeUpRates {
             universal_credit: 0.80,
             child_benefit: 0.93,
             pension_credit: 0.63,
-            housing_benefit: 0.80,
-            child_tax_credit: 0.83,
-            working_tax_credit: 0.67,
-            income_support: 0.85,
             new_entrant_rate: 0.3,
         }
     }
@@ -172,7 +167,6 @@ pub struct UniversalCreditParams {
     pub work_allowance_higher: f64,
     pub work_allowance_lower: f64,
     pub child_limit: usize,
-    pub housing_cost_contribution: f64,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -250,13 +244,6 @@ pub struct ScottishChildPaymentParams {
     pub max_age: f64,
 }
 
-#[derive(Debug, Clone, Serialize, Deserialize)]
-pub struct GrowthFactors {
-    pub cpi_rate: f64,
-    pub gdp_deflator: f64,
-    pub earnings_growth: f64,
-}
-
 /// Convert a fiscal year start year (e.g. 2029) to the YAML filename format
 fn fiscal_year_filename(year: u32) -> String {
     format!("{}_{:02}.yaml", year, (year + 1) % 100)