Can you explain crystallisation?
You have a pot of money/assets inside the pension 'wrapper'. Technically, YOU don't own it - it's held in trust for your (future) benefit. You have a large say in what happens to it, but you don't legally own the cash/assets - the trust does. Even if you go bankrupt the trust remains unaffected (because it's a separate legal entity), and none of your creditors have any claim on it. This is your uncrystalised pot.
Crystalisation can be thought of as act of removing the cash/assets from the trust/wrapper so that they do become yours to access. (This isn't actually the definition, but it's the easiest way to explain it).
At the point you crystallise some/all of your pot you can chose to take 0%-25% of what you crystalise as a TFLS. So if you crystalise (say) £100K from a £1M pot you might end up with....
£900K Still in the uncrystallised pension wrapper/trust/pot which you can chose to crystalise some/all of at a future date or dates.
£75K Crystallised funds in a pension fund account that you can choose to access as and when you want.
£25K TFLS
The £75K can stay with the pension co, and remain in much the same investments as it was in before it was crystalliesd. It only counts as your 'income', and therefore liable to your income tax, as and when you transfer the money from the pension co's account to your own bank account. Whilst it remains in the crystallised pot, it can continue to grow tax free just like it did before it was crystallised. However, you cannot take any more TFLS from the crystallised funds pot EVER - even if it doubles/trebles in value. Every single penny you take out of it will be taxed at whatever income tax rate you pay.
There are many more rules - like you must crystallise everything at age 75 - but that's the jist of it.
One of the 'naughty' but conditionally legal things you can do is crystallise £100K, take the £25K TFLS, and then pay it straight back into the pension. If you're a 40% tax payer, the tax man will gross up your £25K, to (effectively) £42K, and the pension co will add that into your uncrystallised pot. So you end up with £942K in the uncrystallised pot, and £75K in the crystallised pot. Don't touch the £75K crystallised pot, rinse and repeat for a few years......There are rules and regulations on what is called pension re-cycling, but with a bit of planning you can do this perfectly legally.