Cosplay groups don't tend to have their own websites and usually arrange meets through deviantart etc.
In most cases, they are just groups of friends who have made a group and who have similar tastes.
the biggest cosplay group in Glasgow/Edinburgh is CosplayScotland (search on livejournal); run by Yuka and they have monthly meets similar to the GA
as for buying costumes: I'd reccomend looking for commissioners on cosplay island.
The only real cosplay store in UK is tokyotoys but like 90% of online cosplay shops, they're mass produced, ill-fitting cheap crap to be blunt.
With a commissioner, you get a talented person making the costume FOR you; exact measurements, any edits you want, just value for money

same for wigs.
Never buy anything other than good quality wigs.
Party wigs out of Tam Shepharda and the like are rubbish, shiny and impossible yo style and look good.
I've seen countless cosplayers ruin a good costume with a crap wig.
You preferably want a Kanekalon wig
