Since 2004 all Motorcycle helmets have to comply with the ECE22-05 directive and not the BS6658, this is why you will find it difficult to get one in a bike shop.
Some manufacturers still get a BS pass for their lids but as it is no longer compulsory they don't bother as it is an expensive and un-needed application.
You may find some 'older stock' with the BS stamp but beware of buying these, all helmets have a made on date inside the lining attched to the protective shell.
Your best bet will be a car racing shop or at the track, possibly a kart circuit or try Demon Tweeks or somewhere similar.
HTH