Okay, there we have it (Article 34 of the residential Terms, thanks fnarrr):
If we have made a change which is to your material disadvantage, you will not have to pay a charge if you decide to end your agreement early, unless the relevant price terms say otherwise. However, once we have told you about such a change, you must let us know that you want to end the agreement within 10 days. When we make a change that we reasonably believe is to your material disadvantage we will also let you know that you may end the agreement early without paying a charge for doing so.
So clearly I am free to leave now, right? Paying more clearly is a material disadvantage... So I should better send them my cancellation via registered mail, or does the claim number I got after sending an email is sufficient for showing that I responded within 10 days of being aware of the changes? Let's go safe with this one...
Regards,
Mat