Basically you can.
1... 2...
3 - Lobby for a new approach to broadband pricing regulations that make this kind of practice illegal.
I think that to do so would merely bump up prices a few quid, across the board, at least for the 'big 5', as they know that a one-off price rise may cause some churn, but would overall benefit them in terms of income, and lower customer service loads once "leavers" had been dealt with... (and the last to raise their prices might be worried about being overloaded, so I suspect as soon as one upped their fees, the remainder would quickly follow - not be left behind)...
Before anyone suggest this is improbable, I remember a few years ago when Sky raised their prices by quite a few pounds per month, and while there was hardly any competition then, my feeling (and calculations) showed that if they lost 5% of customers they were still going to have an overall increase in income... so a vote with your feet | wallet may sometimes have little or no overall effect, and you might find that re-joining later may be more costly.