I would have thought that any change to any part of the contract that was imposed was cause for the ability to terminate unless the contract specifically permits such changes. After all a contract works both ways and ties both supplier and customer to the rules contained in it.
You may like to examine the contract for any clause which permits Orange to change the terms, conditions and/or charges without your specific agreement. I can imagine you may well find something they can use or rely on as a defence.
In the event you do find something then they would be correct in enforcing the contract and changes/charges made. However, if you speak with a supervisor and express your unhappiness and intent to change supplier you may get a result of sorts. Can but try.