Already generous as in reality favours very few. Withering is an easy way of raising tax revenue without a murmour.

If you are depleting the ISA capital then there's a case for phased crystallisation of the SIPP to pay out just the TFLS to live off instead (and maybe add to ISAs). If you're unhappy with the political risks then you could take the full TFLS and use low-coupon Gilts and/or Premium Bonds to minimise the tax while you stuff your ISA.

If you're able to live off ISA dividends/TR and IHT is a concern and you're likely to die before 75 then there's a case to leave the SIPP untouched. I guess there are political risks with that too.

There are a number of threads on the subject of TFLS / SIPP / ISA trade-offs and it always comes down to personal context and outlook. You may well not want to spell out your income, outgoings, beneficiaries, marital status etc, but you might find your circumstances are covered in some of those posts.

Only until aged 75.

So you can just immediately take out the 25% tax free amount and get free money? I'm assuming this compensates for the remainder which will be subject to income tax...

It will take your platform and HMRC a month or so to add the basic rate tax. The remainder is indeed subject to income tax, so if you're already a basic rate tax payer then it's not exactly the tax dodge of the century - for every £80 pounds you put in, you'll net £85 on the way out, a 6.25% uplift and a max £180 of "free money" per year. Worth having but not life changing :) It's much more profitable if you can make use of the personal allowance in those "in between" years when other taxable income is low.