The biggest problems are that
(A) the uk government is ignoring completely the definition of au pairs as set out in the European court of justice some time ago, which defines au pairs as workers. Why? god alone knows. In fact i am fairly surethat the gov definition that has been linked to was drawn up after the turkish au pair case, so they drafted it knowing that what they were writing was illegal in european law. Because the uk government doesn't define an au pair as either a worker or an employee then they effectively operate outside of the protection of all uk employment law and have effectively no protection whatsoever from exploitation
(B) even if they followed ECJ ruling there are a number of exclusions within uk employment law relating to people living inside a home as a carer, or as part of the family, which render some standard rules - eg NAtional minimum wage, Working time directive - as not applicable anyway.
A whole host of au pair sites will post the 'rules' but most of them are completely wrong in what they say. It is also a complete red herring, by the way, to say that au pairs must not have sole charge of babies, etc, etc. that's not true.
If you followed the ECJ position your au pair would have rights to notice period and paid holidays and some protection from dismissal but still no rights regarding number of hours, nature of work to be performed or pay.
FWIW in the US it is quite normal to have au pairs do full time sole care throughout school holidays of similar levels of 'pocket money'. Am glad most UK employers are a bit nicer than that. Although I do personally know of two cases where the au pair (both girls under 21) has had a row or disagreement and has been thrown out onto the streets there and then
. How some people live with themselves I don't know.