I ask because the HMRC website doesn't specifically state that they can't be from the UK, the gov.uk website states:
An au pair isn’t classed as a worker or an employee if most of the following apply:
they’re a foreign national living with a family in the UK
they’re an EU citizen or have entered the UK on a Youth Mobility Visa or student visa
they’re here on a cultural exchange programme
they’ve got a signed letter of invitation from the host family that includes details of their stay, eg accommodation, living conditions, approximate working hours, free time, pocket money
they learn about British culture from the host family and share their own culture with them
they have their own private room in the house, provided free of charge
they eat their main meals with the host family, free of charge
they help with light housework and childcare for around 30 hours a week, including a couple of evenings babysitting
they get reasonable pocket money
they can attend English language classes at a local college in their spare time
they’re allowed time to study and can practise their English with the host family
they sometimes go on holiday with the host family and help look after the children
they can travel home to see their family during the year