They had arranged with the landlady to pay for a cleaner - losing twenty quid a head out of their deposit seems far preferable to me spending hours there.
Presumably not in the UK. An end of tenancy clean takes hours. My experience as a landlord of a one bed flat with a house proud tenant is that a deep clean (under beds, skirting boards, defrost freezer, oven, back of kitchen cupboards, windows inside and out, calcium deposits off taps, lampshades and lightbulbs, etc) takes two people a day. Yes you can find cowboy firms on the internet who will charge less, but there is no way that they are paying staff anything near UK minimum wage.
DDs flat, 4 students no living room, took effectively 4 people a day, though that was because they had deep cleaned the kitchen and bathroom a couple of weeks beforehand.
Most of my tenants, who tend to be on good London salaries, will opt for a prof clean and simply pay. However we are talking about students. Two of DDs flatmates were not well off at all, indeed one was on a University bursary. When she is older and earning, DD can pay, but it seems useful for her to understand the value of money and to appreciate the work people like cleaners do.