You need to start the leaving process before they'll give you a proper offer
It's 30 days notice and they'll pester you to stay a few times. Find out the cheapest option you can get from uswitch, hotukdeals or similar and then tell Virgin you're leaving. Wait til they call you to persuade you to stay etc.
Or do as I did and literally leave and sign up as a new customer (in wife's name) Ended up having the new service start 5 days before the end of the last one and the guy who came acted like it happened all the time and just switched the equipment over with no hassle.
@musa as a few messages up