There doesn't seem to be a web site. (The George site makes no reference to wedding dresses). Perhaps the George helpline can assist? : 0500 1000 55
Alternatively, there's some basic information, together with a list of stores which were expected to stock wedding dresses, here:
http://www.totallyweddings.com/asda.htm
To check whether they still stock them, phone those stores using the contact details which you can find here:
http://www.asda.co.uk/corp/storelocator_frames et/storelocator_frameset.htm
Chris