I believe there was something in the law that said if you were living in a caravan / mobile home / hotel etc and had no residential home to go to or couldn't return to one if you had one for some reason then it was ok to stay put, worth having a look at the health protection law.https://www.legislation.gov.uk/uksi/2020/350/introduction/made
(3) Subject to paragraph (4), a person responsible for carrying on a business consisting of the provision of holiday accommodation, whether in a hotel, hostel, bed and breakfast accommodation, holiday apartment, home, cottage or bungalow, campsite, caravan park or boarding house, must cease to carry on that business during the emergency period.
(4) A person referred to in paragraph (3) may continue to carry on their business and keep any premises used in that business open—
(a)to provide accommodation for any person, who—
(i)is unable to return to their main residence;
(ii)uses that accommodation as their main residence;
(iii)needs accommodation while moving house;
(iv)needs accommodation to attend a funeral
looking at this it looks like the air b&b you are using is allowed to let you stay.