I've bought kit (including a pistol) from eHobbyasia and had no problems regarding postage / customs* - definitely recommended.
As for UK based retailers, I bought my G&P from Firesupport who were excellent and have very good customer support - I'd say go with them purely based on the service.
* here's a good tip: if you're importing an airsoft gun, put your UKARA no. as an extra line on your address, just to make things easy for the customs people when they see the package.