Sounds about right to me TBH, lets not forget that although the cost of living in London is high, there is no shortage of labour for jobs like cleaning.
The service these people are providing makes it sound more like the agency style of doing business, rather than a complete 'service' that many provide.
At £9 per hour you can bet your life the cleaner will be self-employed or, more to the point, will be SUPPOSED to be self employed (read between the lines there, if you like), with the agreed amount of wage paid to the cleaner by the client (lets say £6 per hour) with the remainder paid monthly to the agency.
It's an age-old situation, and whilst i don't live in London myself, i would find it hard to believe that clients' attitudes to domestic cleaning will be any different to any other parts of the country, that is to say that they won't be 'price shopping' when looking for a cleaner to work in their homes, they'll be looking for someone who they trust and if possible comes personally recommended by someone they know.