I've been to a few diners around London, and this definitely feels like the most authentic.
It's a bit of a trek to get there, but it's so worth it. The diner is inside an old rail car, all red and shiny aluminium sliding with over-sized Pepsi bottle caps on the roof. Inside they've managed to squeeze in a small open kitchen and several booths without making it feel crowded. Having said that there was only one other group in there apart from us. Nostalgic photos cover the walls and along with the jukebox and general decor make it feel real. The food is straightforward but tasty, as well as reasonably priced. My burger was just that, a lonely burger, no salad on the side, but the thick patty made up for it. The brownie with ice cream was delicious and I'm told the milkshakes were good, though maybe not quite up to Ed's Easy Diner standards.
There was one guy cooking and serving, making service a little slow, but we were in no rush and he was very friendly.
The food is good, but Fatboy's is all about the novelty, and is definitely the place for a real old diner experience in London.