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.