Keeping you up to speed

As soon as you phone to cancel they'll start talking discounts, albeit generally modest discounts and with a new 18 month lock in starting.  Or if you do cancel then during your 30 days notice they'll start calling you with the potential for serious discounts (the sort of deals they offer new customers).

If you decide to stay, then check you're happy with the change to terms and conditions that set next year's price rise at RPI + 4%, and because those terms will then be written in the conditions then come next April you won't be able to leave or renegotiate until your fixed term ends.  Moreover that is applied to the undiscounted price, so customers on good deals see much higher prices than the headline level announced.  As a guide, if you negotiate a deal around new customer discount level now, then next April it'll likely go up by 20% or so, that'll you'll have to take.

Or maybe consider Sky.  Price increase this year is significantly lower than VM, and I believe that (unlike VM) they haven't baked a mongo price rise into their broadband package contracts, so if they do increase prices next year you'd have the option to leave or negotiate.