Hey there man... depends on how far out of London you want to go.
Best experience is with my local dealers,
EuroCanterbury in Canterbury, but that's like a 90 minute drive out of London. Pretty easy to find, if you're interested in heading down here drop me a PM. They carry a bunch of accessories, but you may need to order some.
Beadles of Bromley, also in the South East of London are also pretty good I hear.
Otherwise I'd suggest
Alan Day which is in City (so near Westminster, near the Thames as I recall, passed by a few times), I've heard good things but no personal experience.
Hope that helps, and have fun in this green and pleasant land!