Add disability basic income reform option (#866)

Introduces `gov.contrib.disability_basic_income.amount` (£/week, default 0)
and a new `disability_basic_income` Person variable that pays the configured
amount to anyone receiving DLA, PIP, or the UC LCWRA element. Multiple
eligibilities do not stack — the payment is per qualifying person, not per
qualifying programme.

Wires the new variable into `household_benefits`, `gov_spending`, and
`pre_budget_change_household_benefits` so it shows up in fiscal aggregates
and reform displays.

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

Author: vahid-ahmadi

changelog.d/866.md

@@ -0,0 +1 @@
+- Add `disability_basic_income` contrib reform: a flat per-week payment to anyone receiving DLA, PIP, or the Universal Credit limited capability for work-related activity element, controlled by `gov.contrib.disability_basic_income.amount` (default 0).

policyengine_uk/parameters/gov/contrib/disability_basic_income/amount.yaml

@@ -0,0 +1,7 @@
+description: Weekly basic income paid to each person who receives the Disability Living Allowance, Personal Independence Payment, or the Universal Credit limited capability for work-related activity element. Set to 0 to disable.
+values:
+  2000-01-01: 0
+metadata:
+  period: week
+  unit: currency-GBP
+  label: Disability basic income amount

policyengine_uk/tests/policy/baseline/contrib/disability_basic_income/disability_basic_income.yaml

@@ -0,0 +1,61 @@
+- name: Disability basic income — disabled by default
+  period: 2025
+  input:
+    dla_sc: 1_000
+  output:
+    disability_basic_income: 0
+
+- name: Disability basic income — DLA recipient
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    gov.contrib.disability_basic_income.amount: 100
+    dla_sc: 1_000
+  output:
+    disability_basic_income: 5_200
+
+- name: Disability basic income — PIP daily living recipient
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    gov.contrib.disability_basic_income.amount: 100
+    pip_dl_category: STANDARD
+  output:
+    disability_basic_income: 5_200
+
+- name: Disability basic income — PIP mobility recipient
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    gov.contrib.disability_basic_income.amount: 100
+    pip_m_category: ENHANCED
+  output:
+    disability_basic_income: 5_200
+
+- name: Disability basic income — UC LCWRA recipient
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    gov.contrib.disability_basic_income.amount: 100
+    is_disabled_for_benefits: true
+  output:
+    disability_basic_income: 5_200
+
+- name: Disability basic income — non-recipient gets nothing
+  period: 2025
+  input:
+    gov.contrib.disability_basic_income.amount: 100
+    age: 30
+  output:
+    disability_basic_income: 0
+
+- name: Disability basic income — multiple eligibilities do not stack
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    gov.contrib.disability_basic_income.amount: 100
+    dla_sc: 1_000
+    pip_dl_category: STANDARD
+    is_disabled_for_benefits: true
+  output:
+    disability_basic_income: 5_200

policyengine_uk/variables/contrib/disability_basic_income/disability_basic_income.py

@@ -0,0 +1,18 @@
+from policyengine_uk.model_api import *
+
+
+class disability_basic_income(Variable):
+    label = "Disability basic income"
+    documentation = "Flat per-week basic income paid to anyone receiving the Disability Living Allowance, Personal Independence Payment, or the Universal Credit limited capability for work-related activity element."
+    entity = Person
+    definition_period = YEAR
+    value_type = float
+    unit = GBP
+
+    def formula(person, period, parameters):
+        amount = parameters(period).gov.contrib.disability_basic_income.amount
+        receives_dla = person("dla", period) > 0
+        receives_pip = person("pip", period) > 0
+        receives_lcwra = person("uc_limited_capability_for_WRA", period)
+        eligible = receives_dla | receives_pip | receives_lcwra
+        return eligible * amount * WEEKS_IN_YEAR

policyengine_uk/variables/contrib/policyengine/pre_budget_change_household_benefits.py

@@ -39,6 +39,7 @@ class pre_budget_change_household_benefits(Variable):
         "cost_of_living_support_payment",
         "energy_bills_rebate",
         "carer_support_payment",
+        "disability_basic_income",
     ]
 
     def formula(household, period, parameters):

policyengine_uk/variables/gov/gov_spending.py

@@ -56,4 +56,5 @@ class gov_spending(Variable):
         "dft_subsidy_spending",
         "nhs_spending",
         "carer_support_payment",
+        "disability_basic_income",
     ]

policyengine_uk/variables/household/income/household_benefits.py

@@ -58,6 +58,7 @@ class household_benefits(Variable):
         "two_child_limit_payment",
         "scottish_child_payment",
         "carer_support_payment",
+        "disability_basic_income",
     ]
 
     def formula(household, period, parameters):