Hi AlexUK,
6ft 6" is certanly not to tall. I am 6ft 6" myself and have been flying commercially for more then 20 years. Starting on smaller a/c it was a tight fit every now and then, but never impossible. Today I do have the honour of flying the 737, before Airbus, Fokker and Saab. Aircraft are designed around the 95% human being, ranging from somwere below 160cm up to 190,5 cm. In that bracket deviations are allowed (long legs/short torso and otherway around), so if your height is well balanced between legs and torso it shouldn't be a problem. In my case the sum of my lower and upper leg were 130cm, just within tolerances.
Hope it helps! Good luck and enjoy your flying!