Slaw has some good tips, including:
Using a UK SimCard for 3G: While in the UK, I found a large number of mobile phone providers who provided "pay as you go" 3G iPad simcards. I ended up choosing a 1 GB pay as you go simcard from the Three store. The salesperson helped me to install it and set up an app so I could monitor usage. I found it useful to have Google Maps while riding double-decker buses to figure out where I was and where I was going.
