I think there are really only a couple of engineering firms in the UK that actually do the plating. Langcourt Engineering in Weston-super-Mare http://www.langcourt.com/cylinder_repairs_&_replating.htm seems to be the place that a lot of bike shops send them to. I've sent cylinders there directly in the past - no real cost advantage, but it can speed up the turnaround a bit - unless you use BDK or somewhere where they often keep exchange cylinders on the shelf (assuming yours are stock).

The important thing really is to remove all the studs etc and just generally prepare them and pack them well to avoid any hassles/additional service charges. Never had any problems at all with re-plating whether I've sent 'em directly or through an agent/shop.

Cheers

Wb