There may be limits (age 55 min ?) but as far as I am aware you can trigger (cash in ?) your pension any time you wish; but the earlier you do so, the smaller each regular payment.
I know I enquired about 18 months ago and was quoted such a reduced figure that I thought the actuary was having a laugh. Not that I was looking for any 25% lump sum.
Wasn't there also something about the pension/finance company having to offer to do this lump sum option, and it not being obligatory them to offer it ?
As stated it seems you can't get lump sums from DB pensions unless it is in the original agreement. (All mine show a lump sum plus regular pension payment also.) I think you might be able to persuade the finance company to transfer out it to a less attractive "pot" option, if you are "lucky"; and take from that. But I'm unsure that is a good idea. I'm certainly not presently considering it.