Technically, HMRC take the view that all income is taxable. The winnings from gambling, including betting and spreadbetting, do not count as income, but fees charged for the delivery of training, advice, services etc. in connection with such activities do count as income.
Therefore, all income can be required to be declared to HMRC. However, income below a certain threshold level (currently £11,000) will qualify for income taxation at 0% - but its till taxable in their eyes. So, if trading is your sole means of income, even though small, HMRC might take the view you need to declare it. Past experiences reported suggest that HMRC don't want to subject traders and gamblers to income tax, as these people could potentially then claim some tax allowances which otherwise HMRC would not have to grant.
A good idea would be a preliminary consultation with an accountant, or an enquiry direct to HMRC, though their policies do change from time to time without notice.