Odds are that as long as you do not leave the keys behind (usual hint to staff that you no longer need them), and the room isn't listed on housekeeping's Checking Out Today roster, the cleaner will make up the room as though you'll be back, and the next day she gets a bonus (from you) in that she doesn't really have to do anything to mark the room "done" and move on to the next one.
The 50 quid for early checkout would be more a liability against a paid stay, like an airline change fee, than likely assessed as a direct penalty. I don't see them assessing it on a prepaid stay at all, even if you told them (don't!).
Where it comes into play would be:
"I need to leave after 3 nights of this 5 night stay."
"Okay, your rate is 99 pounds per night, so we'll add a line item of 50, and you're off the hook for the 198 you would've paid."