Add Plan 1 BoE margin parameter and use p = parameters(period).gov pattern

- Create plan_1/boe_margin.yaml parameter for the 1% margin
- Update plan_1_interest_rate to use parameter instead of hardcoded 0.01
- Apply p = parameters(period).gov pattern to plan_5_interest_rate

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: MaxGhenis

policyengine_uk/parameters/gov/hmrc/student_loans/interest_rates/plan_1/boe_margin.yaml

@@ -0,0 +1,11 @@
+description: Plan 1 margin added to Bank of England base rate for interest calculation
+metadata:
+  label: Plan 1 BoE margin
+  unit: /1
+  period: year
+  reference:
+    - title: Education (Student Loans) (Repayment) Regulations 2009, Regulation 21(2)
+      href: https://www.legislation.gov.uk/uksi/2009/470/regulation/21
+
+values:
+  2000-01-01: 0.01

policyengine_uk/variables/gov/hmrc/student_loans/plan_1_interest_rate.py

@@ -15,6 +15,8 @@ class plan_1_interest_rate(Variable):
     reference = "https://www.legislation.gov.uk/uksi/2009/470/regulation/21"
 
     def formula(person, period, parameters):
-        rpi = parameters(period).gov.economic_assumptions.yoy_growth.obr.rpi
-        boe_rate = parameters(period).gov.boe.base_rate
-        return min_(rpi, boe_rate + 0.01)
+        p = parameters(period).gov
+        return min_(
+            p.economic_assumptions.yoy_growth.obr.rpi,
+            p.boe.base_rate + p.hmrc.student_loans.interest_rates.plan_1.boe_margin,
+        )

policyengine_uk/variables/gov/hmrc/student_loans/plan_5_interest_rate.py

@@ -14,4 +14,5 @@ class plan_5_interest_rate(Variable):
     reference = "https://www.legislation.gov.uk/uksi/2023/207/made"
 
     def formula(person, period, parameters):
-        return parameters(period).gov.economic_assumptions.yoy_growth.obr.rpi
+        p = parameters(period).gov
+        return p.economic_assumptions.yoy_growth.obr.rpi