Question on Unlocking Phones

Ok, people out there who are cell-phone savvy: I need to know how I can make sure my phone will work when we go to the UK.

I have a Motorola Razr V3. I called T-Mobile and told them I wanted to unlock my phone. They sent me an email with the unlocking code and the following instruction: "Before you start, you must have a foreign (non T-Mobile) SIM card entered into the handset."

I want to test this out before we travel (prob in a few months). Can I buy a SIM card here and try it? If so, where would I buy it, what would I buy?