Virgin have brought back their £10 a month, 300 minutes and 300 texts offer to existing costomers. They put it up to £15 a month a few months ago but now existing Virgin Media customers are able to get it at a reduced rate.
You may need to pull the operators arm who you speak to on the phone slightly but you will get 300 minutes and 300 texts for £10 a month. I believe this offer is only on for a short period of time so if i was you i would be quick.
You can even keep your old number if you request your PAC code off your existing network.
EDIT: Please note this offer is not visable on the Virgin Media website, you have to phone up Virgin in order to get this deal.
EDIT: I also forgot to add that you only have to give 30 notice to cancel the contract. There is no fixed term.
EDIT: BTW if you are going abroad in the next 3 months DO NOT TAKE YOUR VIRGIN SIM WITH YOU. You will be charged around £130 plus calls if you do. It is to stop people signing up, making loads of international calls and then cancelling their direct debit or something along the lines of that.