I've had it once before... Fatburger. Idk if it's in the UK or not. Unfortunately there aren't very many in my area (a long drive to the closest one) so I just ate it once at a food court and it was amazing. They make the best burgers eeeeever.
Aside from that possibly Shake Shack but I don't eat from there often either. I usually just eat burgers that I don't actually like that much but are passable
(Burger King, Wendys or whatever) or from random restaurants/diners that don't really classify so much as "fast food"