It is best to work for a family who have a home in London and a house in the country (it does not matter where). A good example is your compatriot,  Maria Teresa Borrallo, who works for just such a family, the Duke and Duchess of Cambridge, though she is a nanny and not an au pair.

In my opinion, the family is more important than the place. Try to get reviews of the families. London is very busy and more stressful to travel around than anywhere else in England. You might like this though. However, you can find somewhere else which is more relaxing and visit London on days off, instead of living in London. In my experience, it's better to find a good-sized town or small city, than to stay in the countryside.