BIM22016 - Meaning of trade: exceptions and alternatives: betting and gambling - what is a bet?
You should check the other guidance available on GOV.UK from HMRC as Brexit updates to those pages are being prioritised before manuals.
The first, and obvious, question is simply what is a bet? A definition of a bet or ‘wager’ was given by Hawkins J in Carlill v Carbolic Smoke Ball Company  2QB484 and has been followed in later cases:
‘It is not easy to define with precision what amounts to a wagering contract, nor the narrow line of demarcation which separates a wagering from an ordinary contract; but, according to my view, a wagering contract is one by which two persons, professing to hold opposite views touching the issue of a future uncertain event, mutually agree that, dependent on the determination of that event, one shall win from the other, and that other shall pay or hand over to him, a sum of money or other stake; neither of the contracting parties having any other interest in that contract than the sum or stake he will so win or lose, there being no other real consideration for the making of such contract by either of the parties. It is essential to a wagering contract that each party may under it either win or lose, whether he will win or lose being dependent on the issue of the event, and, therefore, remaining uncertain until that issue is known.’