What do you need the broadband for and what alternatives are available in your area?
Many assume that a faster connection = a better connection but if your online activities involve basic web browsing, emails etc. then you should be able to look elsewhere, depending on what alternatives you have available. (I make no assumptions BTW on what your requirements are, just asking the question).
VM will always try to upsell you a bundled package of services. If you keep reliably renewing every 18 months, and the bundle price is competitive to your alternatives, then it can work out well for some people. If you have some other alternatives and you want broadband only and price is a driving factor then renewing with VM for broadband only may not be a good option. (The line that you cannot downgrade to a lower speed having been at a higher speed is, of course, untrue).
Topics on here recently suggest that the best prices only come in once you have put in a cancellation but there is no guarantee at all you will get a call back (and having marketing options on in My Virgin Media is advised to allow the call to take place). So if you do cancel you need to be prepared to follow through with it if needs be.
Any VM offers tend to be 'of the moment' and only last as long as the call back so do your research so that you know what alternatives you have if you have to move. You can then decide there and then to accept any VM offer if it is a good one.
Try this advice from Andrew-G (post #5 below) which might help with the murky business of renewing with VM
https://community.virginmedia.com/t5/Forum-Archive/Why-oh-why-does-loyalty-get-punished/m-p/5226917#M218699