The point you seem to have missed is that the £150 wasn’t a penalty for not following the terms of the railcard. It was the price of a (valid) ticket that they didn’t have.
Whether that is right morally is a different matter, but legally it was two separate transactions that are only related by being between the same parties.
An actual ‘penalty’ on the first transaction could be an excess, a penalty fare or prosecution depending on circumstances.
I think it would be very difficult for the TOC to claim that these two things - which are so inexorably linked - are separate legal transactions. One is clearly the result of the other and hence they must be treated in the same context.
Even if we assume that the argument that "they are not linked" somehow succeeds - there are still two further hurdles to pass for the TOC. Firstly, as no prior price has been agreed for carriage from Carlisle to London, only such a price as is reasonable may be charged (as per
S51 of the Consumer Rights Act 2015). Is £150 a reasonable price for that journey? I would suggest no, it is not - especially in view of the fact that normal ticket prices are much lower.
The second hurdle that arises from the severing of the two transactions is that, looking just at the original Railcard discounted tickets, we are proposing that they have no validity whatsoever if one passenger doesn't turn up. In other words they become void. In my mind, given the circumstances, this is again a penalty for breaching the contract.
As we know, penalties for breaches are subject to regulations. The one in question here cannot in any way be considered to be an enforceable or fair one, because it means that one party (the TOC) can simply decide not to provide any consideration whatsoever if the other party commits a "breach" by not making full use of his contractual rights. That is surely contradictory with the fundamental principles of contract law, that one party has provided consideration (the passenger pays) but then receives nothing in return!