Wow, thank you everyone for all the great info.
I've sent TheCanKey people a request for a place to buy it in the UK, failing that I'll just buy 10 off them directly and hand them out to friends.
I'm very interested in what's being said about shaking the can to ensure the anti-foam agent is dispersed. I have had foaming issues in my WR27-1 (as oil additive said it looked like milkshake behind the sight glass after shutdown) and put it down to the modern BP 2380 I was using being not quite right for the engine. I have just bought a load of AeroShell 500 for it and the GTD-350 so will make damn sure to shake the hell out of the cans this time in case that was the real problem.