But hang on hang on hang on.....
Are you saying it's what HMRC "considers" - as in, they "make up their minds" and not follow a strict law about this??? Supposing two member of the HMRC have different views?!!! It's a joke! These people need to stop being jealous at people getting ahead in life by thinking outside the box! Jealously is not a nice trait.
What I mean is: surely there is a law, a blanket law, that covers this. Betting is tax free. Therefore SB should be tax free. If I stacked shelves in Tesco at weekends for £200 a month, that is my steady, non gambling, fully taxable income surely? So surely the law (and no one is above the law remember, not even our wonderul HMRC) states that my winnings through gambling on any bets is tax free, whether horses, football, FTSE or poker.
EDIT: sorry I'm not buying any of this... I seriously think you would win your case in court or even a European court if it had to go that far. If you are doing something (in this case gambling on financial markets) which clearly states in all literature: "profits are tax free in the UK" - which is does on every website/piece of paperwork within the realm of SB - then you have it set in stone as far as I can see and you would have to win this in court. If this wasn't the case, the SB financial industry would not exist as it holds no use to anybody.