We pay have paid our au pair around £65 per week for 25-30 hours, plus 1 night a week babysitting (which rarely happened, tbh because we didn't go out so much!).
We are not in London but are in a village close to a large-ish city (40 mins from London by train though). We didn't have any probs finding someone because whilst many au pairs do want to work in London, there are also many who request rural or smaller towns.
In addition, our au pairs have had gym membership (which works well for taking children swimming etc too), £10 worth of phone calls on our home phone and a PAYG cell phone provided for UK use only. Usually, we have offered a percentage refund of top-up costs depending on how much they have contacted us with it.
Our au pairs have not driven our cars whilst with us (not necessary for school run etc). We haven't found ourselves eaten out of house and home . The one thing we found with one of au pairs was a massive increase in our water bill. She would shower for approximately 30 minutes every evening and she really wasn't a high maintenance girl, iykwim, but had some serious body image issues.
We usually offered to take her with us on family days out (which we would pay for) but sometimes we wanted time for just us and they have always been cool with that.
Your expectations of an au pair seem totally inline with ours and we have always had plenty of interest in our ads. Agree with Millarkie re: light cleaning. Ours have done the ironing but ime have always taken so long to do just a few things that I would rather they were whipping around with the hoover and let me do the ironing!
FWIW, we have had 4 au pairs. 2 were brilliant, 1 was awful (sent home after 2 weeks) and 1 was so-so: more a personality issue rather than not being a great au pair per se.
Our au pairs have always attended local language courses run by a private college. We offer to refund cost of course upon successful completion as an incentive