Spread betting is seen as a type of gambling and is not taxable, unless it is part of a trade.
BIM22015 advises ‘There has been no decision of a court in Australia nor, so far as I am aware, in the United Kingdom where it has been held that a mere punter was carrying on a business.’ 'However, an organised activity to make profits out of the gambling public will normally amount to trading'.
BIM22015 - Meaning of trade: exceptions and alternatives: betting and gambling - introduction
You will need to decide, whether you are trading or not. Eg. are you spread betting in order to make a living, you could be considered to be trading.
BIM22020 advises "To be taxable, the spread betting wins must come not merely from an opportunity presented by a trade, they must arise from the carrying on of that trade. Whether or not a particular spread bet is taxable will depend on the terms of the contract and the economic substance of what is done".
BIM22020 - Meaning of trade: exceptions and alternatives: betting and gambling - spread betting