I am pretty sure the law is roughly as follows. Any change to your contract which would cause material/financial detriment means you have the right to cancel your contract.
If you are barely using 100mb each month, this change is not going to have any affect on you. If on the other hand you frequently exceed 500mb a month then this is either going to cost you extra and hence cause you financial detriment.
Be weary though, if you cancel your direct debit T-Mobile can file a non payment which can be a pain to get off your credit report. Not the worst thing in the world - but something to consider.