In reply to hpil:
I have done this several times for both University and Scout groups.
Work out what you want and then send it to a few shops for a quote, with a covering letter explaining circumstances. Don't try and get into a bidding war.
Jenny C's suggestion to contact Cotswold and GoOutdoors are pretty much your best options, though. Cotswold used to have a contract price list for big orders - not sure if they still do/what minimum order value is these days. Both will almost certainly do 15% off shop prices, managers/head office may have a bit of discretion for a bit more.
I'm sure you're aware, but once you get your gear, make sure you start up a use log for it and make sure it's filled in.
I did have one run-in with a local shop (now long gone!) who took my requirements for a Scout group, said no problem and then tried to supply completely different items, many of which were brands I'd never heard of and weren't suitable for group use!