Phone Virgin Mobile on 0345 6000 789 from any other phone. If you're calling from abroad, you can use +44 7953 967 967.
Report the SIM as lost/stolen and ask for it to be blocked immediately so that nobody can run up a bill on your account. Ask them to post a pre-activated replacement SIM to the UK address you have registered with them and they should post it free of charge. Phone now and it might get posted today. There will be a charge if you want it couriered.
Make it clear that only the SIM was lost/stolen so there is no need to blacklist your phone.