If you go on to the Home Office site, there is a very specific list of guidelines as to what constitutes an au pair and the main things (apart from the very limited list of countries outside of EU countries which qualify) are that au pairs must be between 17 and 27, not intending to stay in the UK for longer than two years and should only be working maximum 5 hours a day and have 2 days free per week. You are expected to treat them as part of your family and apart from accommodation and food, pay them pocket money - 55 pounds per week being a sort of recommended minimum.
Translated into real life, this means that if you are using the term "au pair" in its correct sense, you cannot depend on them for lengthy spells of childcare if e.g. you are working fulltime. Their childcare experience is limited and as someone else said, what do you expect for that amount of money?
Of course, lots of people bend the rules, pay their au pairs something extra and leave them in sole charge, if not fulltime, at least some of the time. Hey, if that's what you are happy to do - fine.
Much as I like the thought of being able to have cheap childcare for my 2 ds, the reality is that I am paying a nanny most of my salary even though ds1 is at school until 3.15 and ds2 is at pre-school each morning until 12.15. However, the remaining hours are just too many for me to consider having someone inexperienced looking after them and I have to wonder how much enthusiasm, care and experience I could expect for my 55 pounds?