The U.K. has strong laws that taxes and charges must be shown in the headline rate for hotels. They cannot advertise a price plus resort fee or service charges. The price cannot even be advertised excluding VAT (tax).
Yet many hotels in London and the surrounding area have a workaround for this. They’re adding a 5% ‘service charge’ to bills on top of the room rate which isn’t advertised or even disclosed during the booking process. The trick is that it’s a discretionary (optional) fee, which is to say that the guest can ask to have the charge removed.