There are several brokers offering financial spread bets to non UK residents. As their product ranges and Costs differ quite significantly, one has to check individually which fits best.
ETXcapital (they have forbidden me to arbitrage them any further)
SpreadCo (they offer interest on your balance)
SpreadEx (they might even offer you credit lines)
and
CityIndex which might match the product range of IG and CMC (which do not cater non UK residents) at best.
Most of them even offer referral Programmes.
As long as they are FCA regulated with Money insurance of 50k and Access to the Ombudsman, I do not care too much about the quality of the individual broker.
Ad Cantagril: the tax treatment is not the only advantage of a spread bet; spread bets can include a free currency hedge, and if you prefer to hold your positions for longer durations; most Forward/Futures contracts are offered as spread bets