Spreadbetting is not subject to tax in itself, so your profits do not even need to be declared to Inland Revenue. The advice from my accountants is that the IR are reluctant to treat traders as self-employed persons, even if they support themselves from profits and even if they have no other income and work all hours on trading. If your profits become significant, I think it would be prudent to check again with a tax advisor (not the IR at first), as tax regulations do change every year.
Sask - I admire your confidence, but are you really going to be able to support yourself on SB profits? If so, please let me know how, as my job is rather irritating at times.