If you held a ticket from London to Thetford via Cambridge then you are expected to go via Cambridge. This can be done from Stevenage, but not from Peterborough. I'm also surprised that London to Thetford via Cambridge was accepted via Norwich as that also fails to go through Cambridge, unless you'd missed the last train that way and were given permission.
I'm unsure who (or whether) you should try and claim delay repay from becase it seems as though the origin of your problems was trying to use a ticket on an invalid service.
Given that the break of journey would effectively result in two notionally seperate journeys, wouldn't this result in a scenario where delay repay could only be claimed for one of those notional journeys given that you can only claim Delay Repay once per ticket? Setting that aside, you make a fair point for Delay Repay purposes.
However as has previously been established on this forum, (correct me if I am wrong) it is possible to board a train with a ticket which is valid on a different route and pay the excess fare onboard or be issued with a zero fare excess ticket if there is no excess fare to pay. On top of that I still intended to pay the excess fare at KGX prior to travelling if possible but it was not possible for me to do so due to the station being closed (although I did later learn that it had an open door somewhere and a phantom service operating to PBO) which meant that the ticket office would be closed anyway and that therefore paying an excess fare would be impossible, unless I was expected to do so at LST in which case I would end up missing the next train and therefore (at least arguably) voluntarily breaking my journey!
That would potentially invalidate part of my delay repay claim which is not what I want so as I'm sure you can appreciate it is a catch 22 and whatever you do might well be wrong! I had every intent to pay the excess fare onboard the train and had even sought out the conductor prior to the train being dispatched but as I left it a bit too last minute, he had other duties to attend to i.e. dispatching the train so having let him know about my situation (and even produced my tickets) I went to find a seat and waited for him to come and find me, he knew that I'd be in First Class as I told him and even showed him my First Class ticket after he questioned whether I had one (presumably as I wasn't dressed in accordance with the unofficial "dress code").
After the guard had dispatched the train, he soon found me and asked to see my ticket. I produced my ticket for the second time and asked him to sell me the change of route excess (which if he had done would've likely eliminated at least part of the problem as I could've then at the very least made a Delay Repay claim from GA given that the service itself was delayed). He responded by asking me to hand it over, I did that and he then took the ticket before marking it with his pen and simply returning it back to me without selling me an excess fare ticket. On top of that, he did not even endorse the ticket for travel via Norwich or issue me with a zero fare ticket (because he didn't think anyone else would check it). However I feel he should've done one of those things.
In light of that, I feel that I might well have to simply appeal to Greater Anglia's (GA's) good nature and try and ask for a full refund of the fare paid on the basis that whilst they weren't technically responsible for the situation that I found myself in, because the service that was cancelled (and which I originally intended to use) was an LNER service and the ones after that were GTR services; but in spite of that, the actions of a member of GA staff (specifically the guard in question) lead to an (at least arguable) breach of contract on the part of them, the company, which in turn resulted in a situation in which I travelled without a valid ticket (but with the express permission of a member of GA staff) and was therefore technically not eligble for Delay Repay. This should therefore mean that I have an arguable case that I am actually entitled to Delay Repay just as if I had been (correctly) sold a "VIA NORWICH" excess fare ticket or issued with a zero fare ticket on that basis. Am I correct?
I will add that the cancellation occured on 09/08/2019 which was the same day on which a combination of power supply problems and numerous broken down trains (which resulted from the power supply problems and which in turn lead to people trespassing on the line which made things even worse and added fuel to the fire) caused numerous trains departing both St Pancras International and KGX to be cancelled, which included all of the services that I could have caught to either PBO or Cambridge (CBG). On top of that the station was officially closed.