yes ak169 u need visa. I guess u have to go through custom again in london so u might need full visa. It doesnt matter wen u got green card. If u have american citizenship or british cizitenship u r fine but other all ppl need visa, whether transit or full i don know but u need to talk to airlines first to b confirm or jus call british embassy n ask wat question u got. they might give u more clear n correct information. thanx