That would cost the landlord/employer more though - which is exactly why they want to exploit someone this way instead. They say it's 3 hours babysitting a weekday, so assuming it's a 20 weekday month that's 60 hours. At £11.44 an hour (minimum wage from April) that's £686.40, so they are charging, in cash and labour, £1086.40 a month for a single (pretty crappy looking) room as lodger in a family house and as a Monday to Friday let. Even in Clapham that isn't market rent.