OK - not sure which country you're in now but let me assume it's the US. In the US there are two models of iPad available - the AT&T and the Verizon model. Only the AT&T model will work.
You can easily buy a microSIM in the UK for your iPad while you're here. The UK has the most deregulated mobile phone system in the world probably!! Just walk into any mobile phone shop - they're everywhere and ask for a PAYG (pay as you go) microSIM - pay your money - cash or credit card (all types accepted) and, within a couple of minutes you're out the door and surfing the net. Unlike some countries, in the UK they don't want to know anything about you - they just want your money!!
Personally, I'd recommend the mobile operator '3' (yes, their name is just a number) because their network is 100% 3G - i.e. no slow GPRS or EDGE and they have great plans that suit casual short-term usage.
But the bottom line is that, in the UK, competition is very very fierce between the many many operators - probably close to a dozen - and so the prices are very keen. I have an unlimited '3' contract - their so-called 'All you can eat' offering.
I hope you have a great visit to the UK and visit Scotland, where I live, if you can. It's even turned out sunny again today!!
Tim