My experience is that people are always much happier
to give "home"
(ie. in the UK) because it's easier/cheaper/cooler.
So while the lack of tax-deductibility status might hinder the first
year, I am convinced that Wikimedia UK will start off with a lot of
money. Actually, you should be ready for it and start thinkng about
cool projects to channel donations asap.
If nothing goes wrong (and I can't see why it would), tax
deductibility will take about 2 weeks after incorporation. Registering
with the charities commission takes longer, but that isn't required
for the tax status, it's just required if you want to keep the status
after your annual income goes over £5000.