Phonecard in the UK

There are mobile phone shops on every high street in the U.K. Go into any one and ask for a pre paid sim card with a data package. If you have an existing Belgian mobile, it is "unlocked" already, so you just take out the belgian sim card and put in the U.K. one. There may be some small changes to make in the configuration settings in order to use data (surf the net), but if you have a relatively new phone, it should either do this autmatically, or it should be relatively simple to set up the configurations and just switch when you are using the U.K. SIM or Belgium SIM. I have a Nokia E5 and use Proximus contract here in Belgium and Vodaphone pre paid cards when I am in the U.K. and it works fine.