BIM22017 - Meaning of trade: exceptions and alternatives: betting and gambling - the professional gambler
You should check the other guidance available on GOV.UK from HMRC as Brexit updates to those pages are being prioritised before manuals.
The fact that a taxpayer has a system by which they place their bets, or that they are sufficiently successful to earn a living by gambling does not make their activities a trade.
The case of Graham v Green  9TC309 concerned a man whose sole means of livelihood came from betting on horses at starting prices. Rowlatt J says at pages 313 and 314:
‘Now we come to betting, pure and simple… the man who bets with the bookmaker, and that is this case. These are mere bets. Each time he puts on his money, at whatever may be the starting price. I do not think he could be said to organise his effort in the same way as a bookmaker organises his. I do not think the subject matter from his point of view is susceptible of it. In effect all he is doing is just what a man does who is a skilful player at cards, who plays every day. He plays today and he plays tomorrow and he plays the next day and he is skilful on each of the three days, more skilful on the whole than the people with whom he plays, and he wins. But I do not think that you can find, in his case, any conception arising in which his individual operations can be said to be merged in the way that particular operations are merged in the conception of a trade. I think all you can say of that man … is that he is addicted to betting. It is extremely difficult to express, but it seems to me that people would say he is addicted to betting, and could not say that his vocation is betting. The subject is involved in great difficulty of language, which I think represents great difficulty of thought. There is no tax on a habit. I do not think ”habitual” or even “systematic” fully describes what is essential in the phrase “trade, adventure, profession or vocation”.’
This shows that having expertise or being systematic (‘studying form’) is not enough to create a trade of being a ‘professional gambler’.
Some ‘professional gamblers’ do carry on a trade, for example, where they receive appearance money for appearing on television programmes. They are providing a service to a customer (the television production company) for reward. Whether their gambling winnings are proceeds of that trade would depend upon the facts.