If you've already worked for the family in a household that they regularly used for themselves for at least one year then you could apply for a domestic workers visas.
If this isn't the case then it's pretty much impossible to come here to work as a nanny.
The first problem would the family actually becoming licensed to sponsor a work permit.
"You must meet the following requirements to become a licensed sponsor:
you must be a genuine organisation or sole trader operating legally in the United Kingdom;"
