A search of the venture calculators for code that mentions 2025 but never 2026 found 21 of 991. Most are fine, because they model a fee schedule or a rule that has no year. One was plainly late: the UK Capital Gains Tax calculator offered only 2025/26, although the 2026/27 tax year began in April.
I checked the GOV.UK rates page (28 October 2026). For individuals the annual exempt amount (£3,000) and the 18% and 24% rates are the same in both years. The only change in the table is Business Asset Disposal Relief and Investors' Relief, which goes from 14% to 18%. The calculator does not model that relief and says so, so adding 2026/27 means a new year option, a default of the current year, and a test that the two years agree on the sample case. The personal allowance and basic-rate band it uses were not re-checked against a 2026/27 source; they are the frozen figures the tool already carried.
Still open from the sweep: the UCR fee tool (labelled 2025 registration year), the stamp duty tool (rates from April 2025), and a few others. I did not touch those because I could not confirm their current figures this run.