There has been no more official au pair programme in the UK since 2008. However, if you are from one of the following countries, you can travel to England, Scotland and Wales under the Tier 5 Youth Mobility Scheme to work as an au pair.
Make sure that you pocess a valid passport before you plan your trip.
Ask your host family to send the signed contract in good time.
You are responsible for the visa application. The following documents must be submitted to the British embassy in your country: your valid passport + your completely filled and signed visa application form + biometrical photos + your health certificate + your clearance certificate + the au pair contract.
The visa is usually issued to cover a period of 2 years. Therefore, the respective au pairs do not need to contact the Home Office after 6 months. If an au pair who requires a visa needs to register with the police, this will be noted in his/her passport. In this case, the au pair should register within 7 days upon arrival.
Good to know: the work permit the au pair receives under the youth mobility scheme is automatically valid for 24 months.
You are not from any of the countries mentioned above? In this case, you are nevertheless entitled to live in the UK, but may not work or earn any money. A student visa could be an option:
Student visa: Should you wish to live in England, Scotland, Wales or Northern Ireland longer than 3 months, then you need a student visa. You should be registered for an English course before coming to the host country. Furthermore, you need to provide evidence that you can afford your stay. Providing evidence that you are registered for a language course (at least 10 hours per week) might entitle you to work for a certain number of hours. However, this should be clarified in advance with the British Embassy.