Add 2026 student loan interest rate thresholds and clarify uprating methodology

[MaxGhenis]

Summary

The Plan 2 student loan interest rate thresholds (lower and upper) are missing the announced 2026 values and need clarification on uprating methodology for future years.

Current State

The policyengine_uk/parameters/gov/hmrc/student_loans/interest_rates/plan_2/ directory has:

• lower_threshold.yaml: Values through 2025-09-01 (£28,470)
• upper_threshold.yaml: Values through 2025-09-01 (£51,245)

Missing Data

GOV.UK has announced the 2026-04 values:

• Lower interest threshold: £29,385 (same as repayment threshold)
• Upper interest threshold: £52,885

Source: https://www.gov.uk/government/news/student-loans-interest-and-repayment-threshold-announcement-for-plan-2-and-plan-3-loans

Uprating Methodology Question

The Budget 2025 froze the Plan 2 repayment threshold at £29,385 from 2027-2030. The documentation in lower_threshold.yaml notes that "This is the same as the Plan 2 repayment threshold."

Questions:

1. Should the lower interest threshold also be explicitly frozen for 2027-2029 to match the repayment threshold freeze?
2. The upper interest threshold has no announced freeze - should it be set to use RPI uprating for future years?

Historical Analysis

Looking at historical changes vs RPI:

• 2025: +2.9% (lower), +3.3% (upper) — closely matched RPI (~2.7%)
• 2026: +3.2% (lower), +3.2% (upper) — closely matched RPI (~3.2%)

When not explicitly frozen, the thresholds appear to track RPI reasonably well.

Suggested Changes

1. Add 2026-09-01 values:

   • lower_threshold.yaml: £29,385
   • upper_threshold.yaml: £52,885

2. Add frozen values for lower threshold (2027-2029) to match repayment threshold freeze

3. Consider adding uprating: gov.economic_assumptions.indices.obr.rpi metadata to upper_threshold.yaml for years beyond the announced values

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