Ok well thats a tricky one as most are “offshore” in tax havens. Littlewoods who are based in the Isle Of Man for example.
Also i think there is a distinction here between where the affiliate programs are based and the actual casinos. Its possible that the UK bookmakers are UK based as tax rules have been made easier here for gmabling: aside from Littlewoods you could check out William Hill, Ladbrokes, Coral, Victor Chandler and also worth trying is Virgin Games.