If you fail to renew at each and every occasion your VM minimum term expires, and if you do not get a highly competitive renewal price each and every time, your VM monthly subscription will climb rapidly as a result of annual price rises.
If VM is incapable of providing the service(s) you want, at a price that is acceptable to you, then the next step you need to follow is obvious.
Once you have put in your cancellation, and if you have marketing options turned on in 'My VM', you may get a call back with a final offer from VM.
From many similar topics on here, VM seems to be fairly disinterested in retaining broadband only customers because VM's aims are to sell you a bundle of services.
You will get a discounted rate as a new customer with a different provider. Once your minimum term has expired elsewhere, you could return to VM (if you wanted to) and you would be a 'new' customer once again and get VM's introductory discounts.
Swapping providers can be annoying and time consuming but, if price is your key driver, then this is what you have to do. This is how OFCOM envisages the telecoms market is supposed to work.