A couple of options - the first is a list of online stores that I've placed 100+ orders with over the last two years, the second is a bike service company who collect and deliver:

1. The big e-Tailers: Wiggle, CRC, Rose Bikes, Sigma Sport (great for rare stuff, I've found), Tredz, Merlin Cycles, SJS Cycles, Evans Cycles (now defunct), Tweeks Cycles, Cyclestore.co.uk, Leisure Lake Bikes and Triton Cycles.

2. You could use a service company like Havebike. They are London-centric and will collect your bike, service it (or do any work that has been agreed and costed beforehand) and deliver your bike back to you. They only operate within the M25 region.

You could also try eBay UK and Amazon UK.