I'd agree with avoiding 3 and T-mobile. I couldn't get the 3 to work on PAYG on my Mac, and my brother had no end of signal problems with his T-Mobile dongle (monthly sub, not PAYG) dropping signal - and that in a North London location!
I have nothing but praise for Vodafone. My mobile gets signal where others don't, and a work colleague has a Vodafone 3G card for his PC that works everywhere - including a very cold remote coastal factory in Finland! HTH
Now, if only you could get an iPhone on Vodafone network...
