How HMRC measure the profits of different trades
There is a section of HMRC’s manuals that deals specifically with how you can view the rules for measuring profits of specific trades. The list includes over 50 different trades as diverse as actors, athletes, barristers, bookmakers, motor dealers, care providers, doctors and dentists, financial traders, marine pilots, missionaries, pawnbrokers and subcontractors. The index can be accessed at https://www.gov.uk/hmrc-internal-manuals/business-income-manual/bim50000.
For example, HMRC's guidance regarding actors and other entertainers says:
... accordingly, performer’s/artist’s earnings will be liable as the profits of a profession in many cases. The sort of engagement where an employment and PAYE may be appropriate, is more likely to be in circumstances where a performer/artist is engaged for a regular salary to perform in a series of different productions over a period of time, in such roles as may be from time to time stipulated by the engager, with a minimum period of notice before termination of the contract, as was Mr Hitchen in Fall v Hitchen. This would apply for example to permanent members of some orchestras and permanent members of an opera, ballet or theatre company. An employment and PAYE would apply in these cases regardless of the receipt by the performer/artist of other income correctly chargeable as profits of a profession.
Source: HM Revenue & Customs | 01-07-2019