UK has many cheap 'pay as you go' sim deals from most carriers, as do most countries, i.e. Spain, Ireland, etc. Shop around on arrival, T-Mobile and 3 mobile are very good IMHO.
Verison iPads will work in Europe but only on 3G as USA uses it's own bandwidth for 4G and this does not match European standards.
Oh, and might be wise to bring a SIM cutter along (don't cost much) as not every carrier has the best deal on a microSIM. One quick cut and a normal SIM card will fit