The VM sales agents will continue to stonewall you by only offering bigger/more expensive packages.
If you want to downsize, and cut costs, start looking around at what the competitors are offering. That is the only relevant information you need when trying to negotiate with VM.
Do your research into that first of all and try some of the suggestions from Andrew-G at post #5 in the link below
https://community.virginmedia.com/t5/Forum-Archive/Why-oh-why-does-loyalty-get-punished/m-p/5226917#M218699
Your success depends on your negotiating skill and how willing the sales agent thinks you are to actually leave.
If you still get nowhere, give your 30 days notice to cancel as per advice from Cardiffman282. Have marketing options turned on in 'My VM' and you may get a better retentions offer although there is no guarantee at all such a call will come through. So, if you do cancel, you need to be able to follow through with it if VM fails to come back with an acceptable offer.
VM's offers tend to be 'of the moment' and no longer there if you decide to think about it and call back. Having all the facts and figures to hand will help you know if any future offer from VM is good or not compared to the competition.
Finally, make sure you are very clear on the details of what any future package with VM contains (particularly TV channels) as there are regular topics on here where the details discussed on the phone do not come through in the formal offer VM sends through. Check any offer in writing to make sure it exactly matches what you want in any future package before you formally sign up to it.