Normally the rules of a FSP will not allow you to take 25% without starting to draw the pension. There may be a heavy penalty for taking the pension so early, making it not worthwhile anyway. Plus once you are drawing your FSP, you will have of lost this amount from your £10k tax free annual allowance.
As to how much your 25% of the FSP is worth will depend on the commutation rate. As an example, let’s assume that you have a £10k per annum FSP. By giving up 25%, you will receive £7.5k per annum, plus the tax free lump sum.
The commutation rate can be any number, normally between 10 – 20. Again, as an example let’s say it’s 15 – then you will receive £2.5k x 15 (£37.5k) tax free.
Before you do anything with the FSP, you need to find out how much it will pay out at age 55, and the applicable commutation rate from the Pension scheme.
By all means take the 25% of the £100k, plus £10k/year – but I think you’ll be best served waiting until you reach your normal retirement age (as defined by your FSP), given the likely reduction for taking it early.