Why not sue NATS on 2 counts?
Firstly go after them for discrimination because of your height.
Secondly, im sure it was very painful when you hit your head. I assume that as you managed to hit your head that there was no 'Mind your Head sign'. No sign, they are legally liable for your personal Injury - Same principle as slipping when there is no ' Wet floor' sign.
Was the Job offer worded as being "Subject to the outcome of the HSE assesment"?
Refusing you for their reasons appears to me as being the 'height' of incredulousness!
TIO