The LNER "promise" of accepting GTR passengers has completely vanished. When GTR issued a "do not travel" notice because of major disruption a few months ago, I asked both GTR and LNER to accept tickets between Stevenage and London - only to be told they won't because a) the Cambridge stoppers were running (nothing else was!) and b) LNER is a separate company and has nothing to do with GTR passengers!
I can count on one hand the number of times LNER conveyed GTR pax between Stevenage and London. Most of those times it wasn't even advertised!!
Madness, both are ultimately controlled by the DFT and subsidised by the taxpayer who then can't use said train they paying for in multiple ways...
Never mind, I'm sure the wonderful world of GBR will solver everything!