I'm pretty sure all gambling winnings are tax free in the UK?

Thank the power of the horse racing/gaming/gambling lobby.

Many years ago when I lived there, I had an IG Index account - who market themselves as a "financial spread betting" service. At the time, you could buy/sell futures and options with them but it was presented in a way that emphasized that you were "spread betting" - but the mechanics were the exact same and expiries all lined up with the obvious counterparts in the liquid futures space.

So because you were NOT investing but gambling, "winnings" were tax free.

I just googled and they're still going - presumably still offering the same betting "service".

It's funny to see the efforts that scam and pure gambling services go to to try and present themselves as staid and serious "investment" business while IG Index offered access to well-regulated financial markets but kept reminding you that you were betting.