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.