1. Spreadbet firms use OTC products offered by investment banks to hedge net exposure.
2. Do not use spreadbet firms for anything other than longer term* (i.e. at least overnight and preferably longer!) trading. They are very expensive (about 5x to 10x as much as Interactive Brokers).
3. The odds are of course that you are a losing trader, so what difference does the tax exempt status make!
4. If you are a rare winning trader, 10% tax isn't very much and you have an £11k exemption anyway in the UK.
5. One big legit reason to use a SB firm is because your acct cannot support the bigger contract sizes on the exchange. Even then u can use mini contracts etc.
* An excellent use of a spreadbet firm is if you are into passive index investing. No need for ISAs. You can simply buy as much as you like of say ES contracts, replicating the S&P500 and keep rolling them over every quarter. Would cost very slightly more than using SPY but you would have zero tax to pay and unlike ISAs, the amount you can do is limitless! You may be able to do this with individual stocks as well.