Matt Greenall is the proprietor of http://www.ar15.co.uk and although the website isn't incredibly detailed he can provide almost any product for AR's that are UK legal.

Matt is also the membership secretary for the UK Highpower Rifle Association.

Examples of products include:

Full Highpower legal AR-15's

Straight Pull .223 AR-15 Series Rifles in most configurations 

Spare parts for AR and tactical rifles

Barrel Blacking 

Based in Hertfordshire, contact Matt via www.ar15.co.uk