Allowing for your non-financial interpretation of "British", a great list, containing a good many of the greatest films ever made of any nationality. I would echo Jon Kennard's suggestion of "Room at the Top" and maybe also something by Shane Meadows, "A Room for Romeo Brass" or the terrific "Dead Man's Shoes" perhaps? I agree with the 3 Hitches. Watch Frenzy then see the influence on Tarantino - the virulently black humour, the conversations about nothing (Alec McGowan and his wife's Cordon Bleuuagh cooking), the slapstick corpse scene, the juxtaposition of violence and humour. Frenzy might be Hitch's best film.
Cheers Jon, glad you like! Your mentioning the excellent This Sporting Life has made me realise I should really have included it - think I'll have to expand to a top 60! Haven't seen Room At The Top yet though I'm very keen to. As for Alien, like the Kubrick films, the Britishness is up for debate but I've always thought of it as British, filmed in the UK with a British crew and director even though it's heavily US financed.