You'll soon enough find out - I'd guess VM will look to make it as high as possible because of their poor (or negative) profitability. I don't think VM broadband contracts yet include a mandated annual price increase formula, although equally with no formula VM have always put up prices as much as they want when they want, sometimes twice in a year. I wouldn't be at all surprised if VM soon choose to implement a formula in new contracts to track other large ISPs. They'll certainly be looking carefully at the effect of different price rise formulae on different companies, and working out whether locking in an inflation plus rise would be beneficial. If they mandate a formula in contract, then during a fixed-term fixed-discount contract, customers have to take that price rise without any option to leave or renegotiate, and that's potentially a lot of money for VM to help themselves to.
Obviously where the price rise is not detailed in the contract, customers this year can re-negotiate or leave, and VM will have priced that in to their calculations. Those out of a fixed term contract likewise can choose to leave and not pay the increase at all, but because that "out of fixed term" status means a customer is already paying a lot more than VM's average, many will be price insensitive customers who'll keep paying at the new mongo rates. In terms of profitability, VM have always been a two speed business, where at a guess 80% of total margin comes from 20% of the customers, and that 20% will be anybody on standard tariffs, customers with loads of add-ons that they pay extra for such as additional channels, rented pods or boosters, regular pay per view, gadget squad victims, landline extras. Some of the 20% will of course simply be people who re-contracted but didn't manage to cut a good deal, and think that three to seven quid a month represents the most they can hope for.
But none of that applies to me, because my VM contract finishes in a few weeks, and my Aquiss broadband contract has no in-contract price rises or indexation. 🤑