The 25× rule and savings-rate maths transfer; the mechanics don't. The pension access age bridge, ISA-vs-pension sequencing, UK tax on the way out, and where Coast and Barista FI fit — the honest UK version.
FIRE — financial independence, retire early — runs on two pieces of arithmetic: your FI number is 25× your annual spending (the inverse of the 4% rule), and your savings rate sets how fast you get there almost regardless of salary. Save half of what you could spend and you're roughly 17 years from FI starting at zero; save 65% and it's closer to 10. Both transfer to the UK perfectly well. The mechanics around them do not.
UK retirement wealth splits at the pension access age — 55 today, 57 from April 2028. Retire at 45 and your pension, usually the biggest and most tax-advantaged pot, is locked for a decade: your ISA and taxable investments must bridge the whole gap alone. This is the most common structural flaw in UK FIRE plans — enough wealth in total, in the wrong wrappers. The American playbook (Roth conversion ladders, rule 72(t)) simply has no UK equivalent; the bridge has to be built directly.
The 25× rule implicitly assumes withdrawals are spendable. UK pension withdrawals beyond the 25% tax-free element are taxed as income, so a gross pot funds less net spending than the rule suggests — while on the accumulation side, pension contributions (especially salary-sacrificed) are the highest-relief savings vehicle available, and the ISA-vs-pension trade is really a bridge-vs-efficiency trade. And from your late 60s the State Pension — currently £12,548 a year, inflation-protected for life — quietly shrinks what the portfolio must fund in the decades that matter most.
Coast FI: your existing investments alone, compounding at a real (after-inflation) return, reach your FI number by a chosen age with no further saving — from there, work funds only the present. Barista FI: part-time income covers enough that a half-sized draw keeps the plan whole. Both are testable claims about a projection, not vibes — Hundred Summers' Overview card computes them from your actual plan, and its Barista check re-runs the full projection at half salary to see whether every year still funds.
Start with the FIRE calculator for the milestone arithmetic, then test the date properly: the full plan models the bridge, the tax, the State Pension and your real spending phases year by year, and stress-tests the result through simulated and historical markets — including the sequences that broke the 4% rule's worst cohorts.