I suspect you'll struggle in Central London if you want to go via an agency rather than employ someone direct. Don't forget that the agency take their cut (normally £2-3/hr) and need to ensure that the cleaner is getting at least minimum wage on top.
You might be better off asking for direct recommendations from friends/neighbours to employ an individual direct for £8-9/hr.
I live in London and I have been using FastKlean for years now and I am really happy with them. They charge £11 per hour but it is definitely worth the cost. Our cleaner does a brilliant job and the whole team is very helpful.