I only mentioned the reason I found it necessary to leave to clarify that this was not an attempt to "game the system" and to focus the question on whether a Marriott property is allowed to impose unusual terms without notifying guests.
In the decades I have been staying at Marriotts around the world, I have had a need to shorten my stay many times for a variety of reasons (cancelled meeting, reservation at wrong hotel, reservation for wrong end date, flight change, emergency at home, and, yes, uncomfortable room or noisy location). None of those changes ever resulted in charges for un-slept nights though a few resulted in the rate for previous nights being adjusted.
Nothing on the hotel site or in the reservation record indicated that this property's billing practices deviated from those of other Marriott hotels. Perhaps consumer laws are different in the UK. But in the US, this would lead to an expectation on the part of hotel guests that charges would be "usual and customary" (or whatever the correct term is in this context).
ps: I edited my earlier statement to include the all-important "not" regarding regional practices.