The two numbers that decide most of it
Retirement planning in the UK comes down to how much goes in with tax relief, and what rate you can safely draw out afterwards. Almost everything else is detail.
Going in. You can normally get tax relief on up to £60,000 of pension contributions a year. There is a trap attached: once you flexibly access a pension in the ordinary way, that figure collapses to £10,000 a year for good, which is a decision worth understanding before you take a single payment rather than afterwards. Relief is worth most where your marginal rate is highest, and the highest rate on the whole scale is the 62% that applies between £100,000 and £125,140 while the personal allowance is being withdrawn. A contribution made from inside that band is the best-value thing in the UK tax system. The high-earner optimiser works out the contribution that gets you out of it, and salary sacrifice is usually how it is done.
Coming out. The 4% rule says you can draw 4% of your pot in the first year and adjust for inflation thereafter, which means you need 25 times your annual spending. That multiple is unforgiving of small changes: at 3.5% you need about 29 times, so a modest change in the assumed rate moves the target by years of saving. The 4% rule stress-tested on British data, and what £1m actually buys.
Whether a pension or an ISA should come first is the question underneath all of this, and the answer depends on your marginal rate now against your expected rate in retirement. Worked through here.