UC health element reforms can silently score £0: LCWRA amount unreachable in 2026-29, 'rebalancing off' is not pre-Act law, draw gate misses core sims #2427

What is wrong

Two hazards in the UC rebalancing modifier (scenarios/uc_reform.py) can make a UC health element reform silently score £0 or the wrong sign. Both matter for any Budget-day measure.

1. A reform to the LCWRA amount has no effect in 2026–29. add_universal_credit_reform overwrites uc_LCWRA_element as an input in 2026–29:

   • existing claimants get the protected amount, from rebalancing.protected_health_element or the UC Act 2025 s. 4 floor;
   • new claimants get rebalancing.new_claimant_health_element.

   So a change to gov.dwp.universal_credit.elements.disabled.amount (the element's own parameter) does nothing in those years. A single LCWRA claimant in 2027, with the amount raised by £100 a month, shows an unchanged uc_LCWRA_element (£5,259.27) and universal_credit (£16,475.33), on 2.130.1 and 2.126.12.

2. "Rebalancing off" is not pre-Act law. elements.disabled.amount from 2026-04-01 is already £217.26 a month, the new-claimant rate. So rebalancing.active = False pays everyone that rate: the same claimant's element falls from £5,259 to £2,667 a year. A pre-Act counterfactual needs active = False and the 2026+ amount restored to the uprated £423.27 path.

3. The new-claimant draw gate (L140) uses getattr(sim, "built_from_dataset", False), not Impute UC deductions in constituency and local-authority runs #1905's utils.data_source.built_from_data(). A policyengine-core simulation over the UK system therefore draws nobody as a new claimant. policyengine.py builds with policyengine_uk.Microsimulation(dataset=...), which sets the flag, so this is latent.

Fix

• For item 1, either:

  • route an elements.disabled.amount change through to both the protected and new-claimant amounts, as a proportional change; or
  • warn when that parameter is reformed for a year the rebalancing modifier fixes.

  Whichever, document in the parameter metadata which parameters a health element reform must change in 2026–29.

• For item 2, add a documented pre-Act switch, or fix active = False to restore the pre-Act amount path.

• For item 3, use built_from_data(sim).

• Add tests:

  • reforming each health-element parameter moves universal_credit for an LCWRA claimant, existing and new, in 2026–29;
  • active = False gives the pre-Act amount.