Not strictly true, under UK law it tends to be the gas bearing parts of a firearm that are restricted not the furniture. So things like gas blocks and bolts/bolt faces. The exception to this is strangely enough deactivated firearms. You can import a bipod from a Bren for instance but you aren't allowed to modify a deac to be an airsoft gun. Very strange.
Firstly, try this guide
http://www.operationeastwind.com/forum/index.php?topic=2707.0
Secondly, get yourself registered on the WARPAC Cold War Airsoft FB page. I am the Admin on there and semi organise the WARPAC side of things for attendance at the Guman Filmsim Cold War games - which are excellent. Any q's ask.