It seems unlikely to me that we would ever have a need for a "Wikimedia
EU" which would end up being a parent organization to Wikimedia UK.
My thinking on local chapters is that they serve at least these purposes:
First, to provide favorable tax status for the flow of donations. This
is still on a country by country or nation by nation status, sort of,
kind of, even in the EU, although so far I have been unable to penetrate
the mysteries of European law to really grasp the status exactly. :-)
One might suppose, tomorrow's vote notwithstanding, that in the future
tax deductibility across all the EU for a single organization might well
be possible.
Second, and more importantly, as a vehicle for the organization and
support of local community. This depends more on geography than on
nations, and it is likely that very large countries (like the United
States) will end up having multiple local chapters for different regions.
Now, based on these things, it seems that the scope of a UK-ish
chapter might very well depend solely on geography. How to organize it
legally is a somewhat separate question.
I can imagine, for example, that a future Irish chapter might be
incorporated in Ireland for tax purposes, but that residents of Northern
Ireland looking for some friends to hang out with at wikipedia meetups
might well choose to join the Irish organization rather than the UK one,
just because it's geographically easier.
And there could for similar reasons someday be a separate chapter for
Scotland, and the UK chapter ought not, I think, resist one forming if
there are enough people an interest, and if travelling to London for
meetings seems a bit much.
I would imagine that these things will play out differently in different
regions for all the usual diverse reasons that make Wikimedia so
maddeningly complex. ;-)
--Jimbo