BIM22018 - Meaning of trade: exceptions and alternatives: betting and gambling - organised activity
You should check the other guidance available on GOV.UK from HMRC as Brexit updates to those pages are being prioritised before manuals.
An organised activity to make profits out of the gambling public will normally amount to trading. An example of this is the bookmaker.
In Partridge v Mallandaine  2TC179 a professional bookmaker systematically attended racecourses for the purpose of carrying on that activity; he could not legally recover amounts due to him. He was held to be carrying on a vocation and hence assessable to Income Tax on the profits.
Rowlatt J explained the position in Graham v Green  9TC309 at page 313:
‘It has been settled that a bookmaker carries on a taxable vocation. What is the bookmaker’s system? He knows that there are a great many people who are willing to back horses and that they will back horses with anybody who holds himself out to give reasonable odds as a bookmaker. By calculating the odds in the case of various horses over a long period of time and quoting them so that on the whole the aggregate odds… are in his favour, he makes a profit. That seems to me to be organising an effort in the same way that a person organises an effort if he sets out to buy himself things with a view to securing a profit by the difference in what I may call their capital value in individual cases.’
Bookmakers may themselves place bets. Where this is done as part of the trade (‘laying off’), the proceeds or losses have to be taken into account in arriving at their profit.
The key feature is that the taxpayer is likely to be involved in the organisation of the activity. They are not mere punters. They are carrying on an activity where the odds are in their favour.