Thanks for the very interesting discussion, everyone. Clearly there isn't consensus on whether the strategy is morally acceptable. I am quite surprised about this given other methods of creative money-saving are discussed on this board, many of which TOCs wouldn't like publicised. One thing that is apparent is however, is that no-one has cited any specific terms and conditions, which would block this strategy.
Fraud is to make a false representation, dishonestly and with a view to make gain or cause loss. I feel like were it to ever go to court, and it wasn't thrown out immediately, there would be much scrutiny on what it means to be delayed - whether delay refers to the time pressures of oneself or to one's train. It will come down to the exact wording of the forms being submitted, I think. Having a look at Thameslink's form, there is no clear piece of wording that says either way - just "If your journey has been delayed by 15 minutes or more,
we’ll give you compensation under the Delay Repay scheme.". Personally, I think I should have the right to state when my journey begins.
I feel like if the law removes a person's ability to state when they intended to leave and intended to arrive, then you could say that a casual journey with no real time constraints is ineligible for delay repay. It would be insane to demand proof that you wanted to leave or arrive at a certain time.
Also worth bearing in mind that many of these delays are intentionally there due to decisions made by TOCs. A company that strives for 100% punctuality will not make money. A train being cancelled due to lack of staff or signalling issues could have been avoided if there was more investment and redundancy. Is it immoral for a TOC to hire only enough drivers that they can only afford for one of them to be ill?