That applies only to a loss of earnings award.
Any compensation award on other grounds would be unaffected, so if John argued any form of discrimination (for instance) he would receive an amount of compensation for that which is separate to the calculation for loss of earnings (discrimination claims have no upper limit. yay discrimination!)
It used to be the case that the reason for leaving had to be entirely because of the constructive dismissal, but the Court of Appeal ruled in Nottinghamshire County Council v Meikle that the constructive dismissal only had to form some part of the reason for leaving - hence why John can jump ship to another employer and still claim constructive dismissal.
That's not to say he'd win, of course. Constructive Dismissal is a right bitch to prove.