What are some reputable nanny agencies in London and/or Cardiff?

London and Cardiff are the 2 most places I want to be but I know living in the UK is extremely costly, so I thought while I lived there I could also work as a nanny, free room and board, while making extra cash on the side. Also, has anyone been a nanny? preferably someone whose done it in London or Cardiff, whats it like? tips? etc.