I understand where you coming from, noone has really done a legal challenge of the practice.
I just visited the BT retail website to see how things are worded and displayed on their store page.
So the pricing area gives the impression you get the displayed price for duration of contract which is 18 months.
If you scroll down, you see this line.
We may change any prices and terms during your contract. If this affects you, we'll tell you about important changes in advance, and you'll be able to end your contract without any fees.
Now you do have to scroll down quite far, the website is designed for mobile phone users, so a long narrow page with lots of scrolling.
Ofcom would do absolutely nothing, BT are simply carrying out a policy that ofcom has already agreed to.
The ASA would they uphold a misleading advertising complaint? Based on their historical decisions, my opinion is no given what BT are doing is common in the industry and that ofcom also back this practice. A court would probably side with BT as BT do display that bit of information in their ToS. This does not breach any consumer sales regulations that I am aware off.
Basically you can.
1 - put up with the price increase, stay with BT.
2 - move to another provider to try and save the money again you lost on the increase.
3 - Lobby for a new approach to broadband pricing regulations that make this kind of practice illegal.
In short a contract with BT is never really an 18 month commitment either from them or you, because of their regular price increases, customers always have a penalty free opt-out. You could try to contact retentions, explain you effectively not in contract due to the price rise and see if they will offer you a deal to stay, I dont know if their retention staff can offer deals in these situations tho.