Add student loan interest rate parameters

[MaxGhenis]

Summary

UK Plan 2 student loan interest rates vary with income, but policyengine-uk currently only has repayment parameters. We should add the interest rate rules to document the policy even if not actively used in simulations.

Current Interest Rate Structure

Plan 2 (2012-2023 undergraduates)

• While studying: RPI + 3%
• After course, income-dependent:
  • Income ≤ £27,296: RPI only
  • Income £27,296 - £49,130: RPI + tapered rate (0% to 3%)
  • Income > £49,130: RPI + 3%

Plan 5 (from 2023)

• RPI only (no income-based component)

Plan 1 and Plan 4

• Lower of RPI or Bank of England base rate + 1%

Proposed Parameters

Add under gov/hmrc/student_loans/interest_rates/:

• plan_1.yaml - base rate + 1% cap
• plan_2/base.yaml - RPI base
• plan_2/additional_rate.yaml - the 0-3% addition
• plan_2/lower_threshold.yaml - £27,296
• plan_2/upper_threshold.yaml - £49,130
• plan_4.yaml - same as Plan 1
• plan_5.yaml - RPI only
• postgraduate.yaml - RPI + 3%

References

• https://www.gov.uk/repaying-your-student-loan/interest
• https://commonslibrary.parliament.uk/student-loans-and-interest-rates-faqs/