I'm 6'2'' and can stand upright in the middle of the carriage on Central line. The doors however have this annoying curvature so I do have to hunch when I'm standing close to them - which is quite often if you get on at a busy station.
Preach brother, 6"4' here and if I can't get a seat (so always) or am able to stand in the middle of the carriage I'm doing my best hunchback impression.
That said I rarely use the tube these days as it's quicker to get an Uber/Taxi (by a good 15 mins) between Waterloo and my office.
Yup, the roof curves but in the middle it's perfectly fine to stand straight at 6'4" and with some leeway. The time you get stuck by the door, well it's going to be a sad 20 minutes
That matches my recollection of being just barely able to stand straight in the middle of the car, I can't imagine the neck/back/knees of 6'5 londoners. For starters I assume they wear a helmet with a face shield so they don't eat the lintel every time they step into the carriage.
