Add Rural Fuel Duty Relief Scheme (#676)

Apply a 5p/litre reduction to petrol and diesel for households flagged
as being in an eligible rural area via the new
`in_rural_fuel_duty_relief_area` input.

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

Author: vahid-ahmadi

changelog.d/676.md

@@ -0,0 +1 @@
+- Add the Rural Fuel Duty Relief Scheme: a 5p/litre reduction on petrol and diesel for households in eligible rural postcodes, exposed through a new `in_rural_fuel_duty_relief_area` input.

policyengine_uk/parameters/gov/hmrc/fuel_duty/rural_fuel_duty_relief.yaml

@@ -0,0 +1,11 @@
+description: Per-litre reduction in fuel duty applied to petrol and diesel purchased in eligible rural areas under the Rural Fuel Duty Relief Scheme.
+values:
+  2012-03-01: 0.05
+metadata:
+  unit: currency-GBP
+  label: Rural Fuel Duty Relief rate
+  reference:
+    - title: Rural fuel duty relief scheme — Notice 2001
+      href: https://www.gov.uk/guidance/rural-duty-relief-scheme-notice-2001
+    - title: Hydrocarbon Oil (Mileage Allowance for Rural Petrol Filling Stations) Regulations 2011 (SI 2011/2935)
+      href: https://www.legislation.gov.uk/uksi/2011/2935/contents/made

policyengine_uk/tests/policy/baseline/gov/hmrc/fuel_duty/rural_fuel_duty_relief.yaml

@@ -0,0 +1,59 @@
+- name: Household outside the Rural Fuel Duty Relief area pays the full fuel duty rate
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    people:
+      adult:
+        age: 40
+    benunits:
+      benunit:
+        members: [adult]
+    households:
+      household:
+        members: [adult]
+        petrol_litres: 1_000
+        diesel_litres: 0
+        in_rural_fuel_duty_relief_area: False
+  output:
+    # 1,000 litres * 0.5295 GBP/L / 0.5 (statutory) * 1 (economic) = 1,059
+    fuel_duty: 1_059
+
+- name: Household inside the Rural Fuel Duty Relief area receives the 5p/L reduction
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    people:
+      adult:
+        age: 40
+    benunits:
+      benunit:
+        members: [adult]
+    households:
+      household:
+        members: [adult]
+        petrol_litres: 1_000
+        diesel_litres: 0
+        in_rural_fuel_duty_relief_area: True
+  output:
+    # 1,000 litres * (0.5295 - 0.05) GBP/L / 0.5 (statutory) * 1 (economic) = 959
+    fuel_duty: 959
+
+- name: Rural relief applies to both petrol and diesel litres
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    people:
+      adult:
+        age: 40
+    benunits:
+      benunit:
+        members: [adult]
+    households:
+      household:
+        members: [adult]
+        petrol_litres: 600
+        diesel_litres: 400
+        in_rural_fuel_duty_relief_area: True
+  output:
+    # (600 + 400) litres * (0.5295 - 0.05) GBP/L / 0.5 * 1 = 959
+    fuel_duty: 959

policyengine_uk/variables/gov/hmrc/fuel_duty/fuel_duty.py

@@ -15,8 +15,12 @@ def formula(household, period, parameters):
         fd = parameters(period).gov.hmrc.fuel_duty
         petrol_litres = household("petrol_litres", period.this_year) / MONTHS_IN_YEAR
         diesel_litres = household("diesel_litres", period.this_year) / MONTHS_IN_YEAR
+        in_relief_area = household("in_rural_fuel_duty_relief_area", period.this_year)
+        effective_rate = (
+            fd.petrol_and_diesel - in_relief_area * fd.rural_fuel_duty_relief
+        )
         return (
-            fd.petrol_and_diesel
+            effective_rate
             * (petrol_litres + diesel_litres)
             / STATUTORY_CONSUMER_INCIDENCE
             * ECONOMIC_CONSUMER_INCIDENCE

policyengine_uk/variables/input/consumption/in_rural_fuel_duty_relief_area.py

@@ -0,0 +1,18 @@
+from policyengine_uk.model_api import *
+
+
+class in_rural_fuel_duty_relief_area(Variable):
+    label = "In Rural Fuel Duty Relief scheme area"
+    documentation = (
+        "Whether the household is located in a postcode eligible for the Rural "
+        "Fuel Duty Relief Scheme, which provides a flat per-litre reduction on "
+        "petrol and diesel purchased from registered retailers. Eligible areas "
+        "include the Inner and Outer Hebrides, the Northern Isles, the Islands "
+        "in the Clyde, the Isles of Scilly, and specified rural parts of "
+        "Cumbria, Devon and Northumberland."
+    )
+    entity = Household
+    definition_period = YEAR
+    value_type = bool
+    default_value = False
+    reference = "https://www.gov.uk/guidance/rural-duty-relief-scheme-notice-2001"