@user100415 @WeatherVane @Traveller that page (gov.uk/au-pairs-employment-law/au-pairs) is about whether an au pair is considered a worker under UK employment law, which determines among other things whether minimum wage requirements apply. It has nothing to do with whether au pair work can be done by someone who has entered the UK as a visitor. For a non-EEA non-Swiss national, au pair work is permitted only on a youth mobility visa, as noted in Traveller's link.

@CMaster au pair work is usually not covered by minimum wage laws; see Dani's link (repeated in my previous message).

Correct link, missed the edit deadline: gov.uk/au-pairs-employment-law/au-pairs