Wolf Armories
only list UKARA, Defcon only list
UKARA, British Airsoft Club and site membership, Land Warrior only list
UKARA (and implicitly site membership), Crawley Surplus
only list UKARA, BZ tactical
only list UKARA, A2 only list
UKARA and site membership. Although they may very well accept it if you ask.
FireSupport, AirsoftWorld and Weekend Warrior do indeed list Sportsman Association, and/or British Association for Shooting and Conservation (BASC).
They must surely be aware that these are not VCRA defences, neither by the letter nor the spirit of the law - if your interest is in target shooting, it doesn't matter if you have a RIF or an IF, right?
Still, it could be worse. There's one site that just allows you to tick a box saying "Yeah, mate, airsoft and that" (although somewhat bizarrely does want to see proof of age).