Importing from states to UK is pretty easy. I've not had too many problems yet.
Duty is usu about 1 % - VAT is the killer - but of course if you are VAT registered, then provided you have the correct forms generated @ customs (?C37?) you can claim back the VAT.
Shipping of course varies from courier to courier. I've used Bax Global, Fedex, UPS and others in the past - none have caused any trouble.
As for UK vendors - FJ, sounds inc, mjq etc. are the obvious ones for used/'vintage' gear. - I've had mixed experiences with FJ but I know some of the techs that do work for FJ and they, at least, do an excellent job - that's not always been the case though. Got my plate reverb from malcolm - nice chap - pleasure to deal with. Also bought various items from Nick Ryan. He's not always the cheapest but he tends to have the goods! Also look at Gearonline (
www.gearonline.co.uk) - they have some funky bits in occasionally. Andy and Alison are nice people.