Scottish top rate (48%) starts at £116,760: threshold stored as £112,570 instead of £125,140 #2130

Problem

Since #1488 (merged 2026-01-21), the Scottish top rate (48%) threshold in policyengine_uk/parameters/gov/hmrc/income_tax/rates/scotland/rates.yaml is £112,570 of taxable income for 2024-25, 2025-26 and 2026-27. The law puts it at £125,140, so PolicyEngine charges 48% on income that should be taxed at the 45% advanced rate.

#1488 reasoned that thresholds are stored above the personal allowance, so £125,140 − £12,570 = £112,570. That subtraction is right for the lower bands, but not for this one: the personal allowance is fully withdrawn at £125,140 of adjusted net income. At that point taxable income equals total income, so the taxable-income threshold is £125,140. This is the same value PolicyEngine already stores, correctly, for the rUK additional rate (rates/uk.yaml, £125,140).

GOV.UK, Scottish Income Tax (https://www.gov.uk/scottish-income-tax): "Advanced rate £75,001 to £125,140 45%", "Top rate over £125,140 48%", and "You do not get a Personal Allowance if you earn over £125,140."

Effect (policyengine-uk 2.93.0, 2025, Scottish taxpayer, employment income only)

Under the personal allowance taper, taxable income is 1.5 × income − £62,570. It reaches £112,570 at £116,760 of income, so the 48% rate starts there instead of at £125,140.

Every Scottish taxpayer with income above £125,140 pays £377.10 too much (3% × £12,570), and those between £116,760 and £125,140 pay part of that.

Fix

• Set the top-rate threshold to 125_140 for 2024-04-06, 2025-04-06 and 2026-04-06, and correct the comment.
• Update policyengine_uk/tests/policy/baseline/finance/tax/income_tax/scottish_rates.yaml so its expectations come from the bands above.
• Add a test at £120,000 and one at £130,000 (expected values in the table).
• Do the Axiom parity check for the Scottish rate resolution in rulespec-uk.

Found while reviewing PolicyEngine/policyengine-uk-data#533, whose salary sacrifice relief targets classify Scottish relief by bracket. That PR reads the thresholds from the parameters, so this fix changes only which bracket some Scottish relief falls in.