Try and give Virgin Media notice a close to 30 days of your contract expiry as you can, although you can legitimately give notice now VM media staff will try and fob you off and say you have to give exactly 30 days notice (this happened to me).
Call 150 from you landline and choose option 4, be prepared for an onslaught of VM staff trying to offer you a "great deal" at ÂŁ50 more than your currently paying.
Stick to your guns and hopefully they will eventually get the message thst you really are intending to leave, some people also suggest sending a recorded delivery letter to VM HQ regarding your intentions to leave.
The other option is if you moving to another provider go down the On Touch Switch route, this is where you take out a new deal with a different company and allow them to do the hard work. However be warned VM kept blocking my move to Sky using this route, firstly by not allowing my landline number to be ported and then by suggesting I'd taken out a new contract with VM (which I hadn't).
Either way VM will make it very difficult for you to leave.
Good luck