If you are a citizen from any of the "old" EU member countries (Germany, France, Italy, Blegium, Luxembourg, Holland, Denmark, Ireland, Greece, Portugal, Spain, Austria, Finland and Sweden) you can come and work as an Au Pair without any restriction. You don't have to register before you start working.
You will normally have to register under the Worker Registration Scheme if you wish to work for an employer in the United Kingdom for more than one month and are a citizen of: Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Slovakia or Slovenia. The fee for registration is £90, and you will be issued with a registration card and certificate.
If you come as an au pair however, and earning below the lower earnings threshold then it is not necessary to register under the Worker Registration Scheme (and pay the associated fee).
The advantage of registering is that, once you have been working in the United Kingdom legally for 12 months without a break in employment, you will no longer have to register on the Worker Registration Scheme. You can then obtain a residence permit confirming your right to live and work in the United Kingdom.
If you are a citizen of Romania or Bulgaria and want to work as an au pair, you are not required to obtain a work permit. For Au Pair work you have to apply for an Accession Worker Card (Purple Card).
The purple card is issued to you just for the work you do for one specific family (if you change family, your card will be invalid and you will need to make a fresh application).
While working as an au pair, you are not legally allowed to take up any additional emplyment.
Once you have worked in the UK continuously for 12 months, you are entitled to apply for a registration certificate known as a Blue Card – this certificate indicates that the holder can work in the UK without any restrictions.
If you are from the above countries, then you can come to the UK and work in an au pair capacity by applying through the Youth Mobility Scheme. It is a scheme for young people from participating countries who would like to come and experience life in the UK for a period of up to 2 years. If you are a national of Australia, Canada, Japan, Monaco or New Zealand, you may be able to come to the UK under the youth mobility scheme.
You need to be 18 years old or over when your visa lets you come to the United Kingdom (becomes valid) and under 31 years old when you apply; you need to have £1,600 in available maintenance (funds) when you apply.
You will be ineligible if you have:
You cannot apply for the youth mobility scheme if you are already in the United Kingdom.

Our short guide to au pair pocket money should help you work out what to pay your au pair..