Grabbing a sandwich or even a hot roll from the buffet is not enough after a full day away. Yes, the GNER/NXEC restaurant meals were expensive, but as has been said elsewhere - not much different from an average London restaurant and you're travelling at the same time.
In the mid-90s I started to promote the use of Leeds/Kings Cross trains to my work colleagues, and suggesting they use the dining car. This to them was a revelation and led to them being "converted" to rail travel both for business and leisure (even taking their families by train which they had never done before).
One of the ways to lessen the cost of the meal was, of course, to buy a cheap standard return and take your chance by filling up the spare seats not taken by first ticket holders in the restaurant car. I think I only once failed to get a seat. In this way, the cost of the meal was almost zero. This was not only a bonus for me, but the catering department got extra revenue and the staff got more tips. Everyone was a winner.
There was only ever one occasion when this became a problem (during the fag-end of BR days) when a stroppy guard, on seeing my standard ticket said, in a loud voice, "OK, but I want you out of the restaurant by Doncaster!" When Doncaster came, I was still eating my dessert, but the waiter came up to me and whispered "Take your time, he's an old ****!!"
Ok, so now we get "free" food in 1st, but this is very hit and miss and of variable quality - certainly not what anyone can call a "meal".
"Nothing could be finer than dinner in the diner" really is true if you get good food, service and scenery. GNER achieved this in spade fulls. When NXEC eliminated most of their restaurant cars I stopped using them for special days out and I'm sure I'm not alone.