I agree it is hard finding suitable people for cleaning jobs but I wonder why you stick with the self employed mantra ?
Few cleaning jobs actually pass HMRC's guidelines of true self employment and candidates are wary of self employed jobs, believing that an employed position is somehow more secure.
The other thing is your asking for them to provide their own transport and insurances. To be legally compliant, they would, as self employed, be required by your insurance company to maintain public liability insurance of their own at least as comprehensive as yours and for their transport, to have a valid MOT, tax and business insurance. For £7.50 to £9.00 an hour that is asking quite a lot.
If you put two people on full time, would the numbers not work for you to provide transport ?
We also use self employed, although not too often. When working for us they are classed as 'labour only' and are covered by our liability insurances. I would assume this is the same accross the board, although def best to double check with your own insurer.
As for transport, you do have a duty of care to ensure essential things in respect of any motor vehicle used in the course of your business. This includes checking the driver has a current full UK or equivalent driving licence, the car is road-worthy (this is of course subjective and you are unlikely to be an expert, but it's just a common sense check), MOT, RFL and Insurance are up to date and insurance is relevant to type of use. You are not liable to pay for any of this, just to ensure they are in place.
As for hourly rate, £7.50 is def bottom end for self employed. This equates to approx £6.40 employed, but without any of the security.
One last point, to be officially self employed they can't only work for you.