r8sso, Tesco use the O2 network, but Virgin use the T-mobile network.
Having said that, not sure if you can use an O2 sim in a Tesco mobile, but I think that you can use a Tesco sim in an O2 mobile.
As for unlocking a Nokia mobile, you can unlock some easily via a (free) calculator. Try at Unlockitfree to see if it can be unlocked for free.
HIH
LB2