I always joined my au pairs to JAGS gym- there are classes there they can interact with and meet other people. Other gyms are available of course. I also introduced them to other friends who had au pairs. I always took a full day at the start to show them around the local area- join libraries, showed them cinemas and explained cheap nights etc etc. Also i always made it clear that their weekends were their own apart from very occasional babysitting so they could go on day trips, stay over at a friends etc ( I also had a trundle bed so they could have a friend stay over) . Show them facebook groups for au pairs in London. TBH they will probably be keen to shoot off as soon as they can and explore London while they can.