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