You can try to renegotiate with VM and renew at a lower price. Phoning in at 08:00 on the dot when the lines first open will give you the best chance of connecting to a UK agent with least delay. Take the 'thinking of leaving us' menu options.
Before doing so though, check out the comparable prices from other providers in your area so you can benchmark whatever offer VM comes up with.
Success (or not) is really a lottery depending on who you get to talk to, what authority they have to make offers and how well VM is doing against its sales targets at the time you phone. Any 'deals' they offer tend to be one-offs and may not be available if you want to think about it and call back so do your research to know alternative prices before calling to decide if any offer is worth it.
If you move to another supplier you will get new customer pricing and, so as long as speed is not a particular concern for you, you will very likely get a better price elsewhere for BB only, certainly better than £51 p.m. so do some research first to find out what else might be available to you, then (try and) talk to VM to get a better deal.