Well, I was delayed on the high speed, meaning that I missed my connection. I did get to my destination an hour late.
Claiming for the actual delay to your journey is fine.
If your first leg was delayed by say an hour but you managed to get to your ultimate destination only say half an hour late, you have two options in
practice:
1. Claim for the hour long delay to leg one. Let's say this is worth £30.
2. Claim for the half hour delay to the journey. Let's say this is worth £20.
Knowingly doing number 1 when you
know that Delay Repay is meant to be based on the full journey would be section 2 fraud.
Number 2 is what you should do.
However, the word 'knowingly' is key. You can't be committed of fraud by false representation if you didn't
intend to defraud. What this means in practice is the average traveller will 'get away with it' if they claimed for number 1, even if they bought the split tickets in the same transaction with an itinerary because they can just say oh I didn't realise it was the full journey not per ticket (a misconception a lot of passengers probably have). If on the other hand you frequent the fare advice section of a railway forum and your username is a unit of rolling stock number and you've been posting asking questions about which is valid, well you'll lose your credibility arguing you didn't intend to defraud

The odds of it being prosecuted are incredibly slim of course.
There is also potentially some ambiguity at play. The NRCoT say that a journey
may be comprised of multiple tickets. It doesn't actually say that multiple tickets in a single purchase
must be considered a single journey. You could, for example, with split tickets end your journey at the station the tickets split at and that's perfectly valid. The question will ultimately be what did you do in reality. If leg 2 is a flexible ticket and you take a later train, then your case for claiming compensation for leg 1 becomes more plausible.
It's a question of 'having your cake and eating it' - if you regularly take advantage of split tickets and claim delays on the full journey cost, it seems a bit unfair to then decide to only claim compensation on individual tickets in a split ticket journey when it suits your wallet. Like I say though, odds of being caught and held to account are slim; at worst you're likely to just be asked to clarify well you bought these two tickets from a split ticket website as a single itinerary so did you make the full journey or not, and when you say you did use all the tickets, you'll be informed that you'll be getting the lesser compensation.