Go for it then, but I think you will struggle with legal action, you admit that you had foresight and they didn't, you admit the contract was unbalanced but still signed it, the contract stated that unless you ceased training for serious injury or death you would lose £10,000, you didn't but they still gave you it back.
It's quite possible that they have had students as tall as you and you said you have no problem with Cessna's, I'm a little shorter than you but couldn't use the 150 and 152 Cessna's, I could fit in but struggled more with the width and the weight.
Put it another way, if you went and bought a car off a dealer who deals with drivers all the time and you bought the car but couldn't fit in it, who would you think was responsible?