From my research it's actually be advisable not to take the 25% - I'm in a simple world with just a single pension pot.
Huh? Why? It makes no difference when you take the 25% tax free. It's always tax free (well unless/until the chancellor changes the rules). I'm yet to hear a convincing reason NOT to take the full 25% TFLS from defined contribution pots.
The argument is different for defined benefit (aka DB/Final Salary) pots, because you usually have to commute some of the annual income in exchange for a lump sum. In those instances you have to work out the commutation rate. Would you prefer £10K p/a for life or £8K p/a for life and a £25K lump sum?
What I've read is that 25% is for the life of your pension, you don't need to take it in one go.
40% threshold is ~£50k
So pay yourself ~£49k from your pension pots (paying ~20% tax)
Then top up your income from your 25% tax free lump sump, say £15k over the course of the year.
So you get a £65k pension and only pay 20% tax.
You only ever pay 20% tax and over the course of your entire pension that would save far more.
It all depends on what are called 'crystallisation' events.
Say you have a £1M pot. You can.....
1) Crystallise £50K, and take 25% tax free, and then take the remaining 75% as 'income' an pay tax at 20/40/45%. You leave the remaining £950K inside the uncrystallised pension wrapper.
2) Crystallise £50K, and take 25% tax free, and leave the 75% inside the pension as 'crystallized' funds. You leave the remaining £950K inside the uncrystallised pension wrapper.
3) Crystallise all £1M, and take 25% tax free, and leave the 75% inside the pension as 'crystallized' funds.
What you cannot do is crystallise £50K, but take 25% TFLS of the whole £1M pot. You can only take 25% of whatever you crystallise. Once it's crystallised, you can't take any further TFLS from the crystallised portion of the pot.
On the tax point, if you crystallise (approx) £66K, then the TFLS part of that will be £16K5, leaving £50K exposed to income tax. You have a £12.5K personal allowance, so will pay 20% on £37K5, which is £7.5K. Therefore, you will receive £58K5 into your bank account, and pay £7.5K tax - an effective tax rate of 12.8%.