PC from CMC markets was asked this on his q&a trad2win thread. I think he then consulted his in house lawyer and put a reply on his thread.
I should dig up that reply, but i cant be bothered at this time of night.
From memory PC said something along the lines of .. if what you are doing is gambling then SB profits are are not taxable.
So when is Spread betting not gambling?
Im not sure, but i can think of a few grey areas:
1. Arbitrage, say you are exploiting pricing differences across 2 spread bet firms and pretty much guaranteeing a profit for yourself. This used to possible in the early days of internet spread betting.. not so easy anymore.
2. Market making. Paz is quoting some marketing material from Kyte. I assume they have a few profitable ex prop firm traders who are using their spread betting offering via the DMA wrapper. Now if the trader is using a DMA wrapper for Market Making style of trading (eg putting out buy and sell orders very close together at the same time) and the majority of his profits are from capturing the bid/ask spread, then that might not be considered gambling by HMRC, even though a spread bet wrapper is used. (I may be completely wrong, HMRC may have no problem with this at all).
But for the majority of spread bettors, even full time 'sole income' ones, who are just making directional punts, yeah, not taxable.