Just had this exact thing with Sky. Go on a comparison site, find what offers others have, and knock a bit off, thats then your base price.

Ring Virgin and say current climate etc thats too much, Sky have said I can have more for £x. So can I speak to whoever I need to cancel.

Just be nice to them, and keep to the line of i like your service but its just pounds and pence. By the time it gets to retentions they will 100% offer you the same or less than you were paying. You just need to be prepared to follow through and leave if they call your bluff. And even then you will get a call the next day offering some magical deal.

Unless you arent happy like. In which case all the others will have offers on to get you to switch so just switch.