I just got a virgin mobile phone unlocked. Just phoned them and they sent the unlock code (the one your new sim card asks for) in the post. You'll need the IMEI number, which should be inside the phone somewhere. You sometimes have to have had the phone for a certain time or spent more than a minimum amount on calls before they'll do it. Failing that, you can often get most popular phones unlocked by the market stalls who sell phone stuff for less than a tenner.
Once unlocked, you can still use your old sim and you can get a twin sim back for that phone - allows you to use both old and new sim and select either network eg depending on coverage or tariff etc. PM or email me if you want a link to twin sim seller or google for it.