I think the main principle is not to take the mick, isn't it?
And if you must take the mick, try not to advertise the fact on forums like this, as it may steer the debate on compensation for delays in directions that wouldn't favour the passenger. It's one of the only areas where UK rail passengers get a better deal than rail passengers elsewhere in Europe, and if you travel long distance regularly, it helps offset the high fares.
For me personally, there are circumstances where a delay on the first leg of a journey might inconvenience me. I've got a claim in with TPE for a journey from Kings Cross to Hartlepool that included a 50 minute wait at York, with the tickets split at York. I booked it with a 50 minute wait rather than a 20 minute wait because I knew when I got to York station I needed to buy two newspapers, tear out the coupons and buy a Northern day ranger for use the following day - my only chance to do so given that by the time I got to Hartlepool, the ticket office would be closed and wouldn't open again until the Monday.
It is perfectly feasible, therefore, that a delay on that first leg (of say 35 minutes) could have seriously inconvenienced me, notwithstanding that I could still have made my connection. As it was, my train into York was on time, giving me ample time not only to buy my papers and my day ranger but also to get brunch in the station pub. It was the delay on the York to Thornaby leg that made me miss my connection and arrive in Hartlepool just over an hour late. I'm claiming that as a delay on the entire journey from Kings Cross to Hartlepool, which it was. There was no earlier train I could have got from York - I always intended to get the first train towards Thornaby and I did, my destination was Hartlepool and I was delayed about 65 minutes.
But I also had business to attend to at York station which meant, had I instead been delayed on the London to York leg, still made my connection and been able to arrive at Hartlepool on time, I would have suffered non-trivial inconvenience in terms of not being able to buy the ticket I needed for the following day's travel, or at the very least not being able to get anything to eat until I reached Hartlepool. I've always been under the impression the delay repay scheme exists to compensate passengers for delays that cause them inconvenience? I accept, of course, this would have meant adopting a flexible attitude to the question of whether I was making two separate journeys or one through journey, which isn't something I would normally condone, but it was a situation where a 30-35 minute delay inbound to York, with the train to Thornaby leaving on time, would have basically wrecked my plans for the following day even if it didn't literally cause me to miss my connection.
In most cases I quite agree that delays which don't affect arrival time at the final destination shouldn't be claimed for.