Going by my previous statements the commutation rate is around 1:20, so for each £1 of yearly pension you give up you get £20 lump sum, and I always thought it was that!
However going by an ongoing thread
, about someone wanting to take their pension early at 55(current employee), it's actually more like 1:25.
But either will give you a rough ballpark figure.
As has been said, if you're soon to be 55, you can contact the Pensions Service Centre in Sheffield for a quote.