This idea that you cannot choose how you define the journey you were making at the time you were delayed isn't actually supported, in my view, by the NRCoT.
The NRCoT doesn't define "journey". If you hold a flexible ticket without break of journey restrictions, then you might be making partial use of your ticket and so the journey you were making at the time you were delayed may be one of several separate journeys you made, or intended to make, using that ticket.
This may be controversial, but given the extreme padding in the London MCTs and cross-London transfer times, it would be very much in most passengers' interests to define their travel either side of a London terminal as separate journeys.
Consider a passenger who buys a flexible ticket from Crayford to Manchester Stations, gets to Euston in time for the 18:00 (albeit by making unofficial connections), boards the train at Euston before its scheduled departure time and then sits on the train as it picks up a 30 minute delay between Euston and Manchester Piccadilly. The idea the delay doesn't count because you weren't slow enough through London, is just not what the NRCoT says. It sets out an entitlement to compensation "if you are delayed in reaching your destination as a result of a delay or cancellation of
a train service."
To meet that condition, you have to show that if a particular train that you used or intended to use had not been delayed or cancelled, according to the timetable you would have arrived at least X minutes earlier than you did (where X is the TOC's delay threshold). There is absolutely nothing to say you can't take the view that Crayford to Euston was one journey and Euston to Manchester was another journey.
It is also fanciful to describe a claim submitted on that basis as "fraudulent", when the railway hasn't defined the term "journey".
Unless I'm missing something? Where does it say you can't opt to begin a new journey whenever you find yourself at a station before the MCT from your previous journey has elapsed?
Edit: Just to add, I'm not disagreeing with those posters who have advised the OP against attempting to resubmit her claim with revised journey details. Once you've defined your journey by submitting a claim, I do agree that if you then start tweaking it on appeal in order to try to get them to pay, that is potentially going to get you in trouble, maybe rightly so in some cases.