To be honest I'm not sure whether a court would consider you to have accepted the new terms if you didn't write to the landlord and say you didn't. It's possible, and if you started to keep the new rules then I think there's a good chance you'd be seen to have accepted them.
But that aside, whether the landlord can vary the agreement depends on whether the agreement gives the landlord this power, and if it does whether it's phrased tightly enough. Without a variation clause then no, the landlord has no ability to vary the terms without agreement. If theere is a variation clause then it needs to be fair under the Unfair Terms in Consumer Contracts Regulations which means you either have to be given sufficient notice of the change for you to quit the tenancy without loss and the clause has to spell out exactly what kind of change the landlord would be able to make and why. Failing that the landlord's option is always to terminate the tenancy and offer you a new one on new terms.
So the likely answer is no, the landlord can't impose a change in the agreement without your agreement, and to be safe you're best writing to the landlord saying that you don't agree to the change.
Does the old agreement have a variation clause?
