Hello
You pay the cleaner for the work they carry out so if you have a one bed apartment that would be much less work then a seven bed mansion.
Never heard a cleaner working on a commission basis - why would they - if it takes them three hours then it takes them three hours - whatever profit you might make.
Rates per hour vary but in London you will probably pay quite a bit more than other parts of the UK. Cleaning agencies charge more than contractors you hire directly but have the advantage that they have more staff available so probably better for last minute bookings.
Get a few agencies and individuals to come around and see your property and then you can discuss terms with them.