Ah but, comparing hourly rates between paying an individual and paying an agency, isn't a fair comparison... as agencies have all those extra fees. So your £8 per hour may actually turn out to be less cost, than your friend paying Sitters a lower hourly rate (but having to pay booking and membership fees).
Interestingly, the nannies/childcarers who have posted so far, all seem to charge about the same. Those who use nursery staff, also are paying them about the same (though less than a nanny). Plus unqualified people (such as au-pairs) are getting less. Location is also a factor, within London area itself, plus within the whole of the UK. Local market forces will dictate how much a babysitter can get.