Evans tend to have the best ranges in their larger stores. Otherwise,
Decathlon in Reading. Most cycle shops carry a fairly small range due to space limitations. Best is to find out where the shops are and visit.
Lots of good size stores in London if you just hop on the train - Evans, Cyclesurgery, OnYourBike,
Condor if feeling rich, Dcathlon at Surrey Quays at the other end of the price scale!