I could be wrong, but I vaguely recall reading something that "England only" (or probably England-and-Wales) was being looked at, as it potentially simplified matters from a legal standpoint.
Yeah, the current Plan is to incorporate under England-and-Wales law, but we'll be for the whole of the UK.
UK is the working name for the chapter at the moment, though this does unfortunately exclude Ireland, which might not be large enough to get its own chapter yet, so I think there is still room for changes to be made to incorporate them.
This was discussed at http://meta.wikimedia.org/wiki/Wikimedia_UK. The suggestion of using "WISE" (Wales, Ireland, Scotland, England) was made at http://meta.wikimedia.org/wiki/Talk:Wikimedia_UK
Angela.