LNER seem to be branching out a little, lately:
>https://www.lner.co.uk/news/lner-cleethorpes-test-train-takes-to-tracks/...sending a train 'the long way around' last month from the depot at Doncaster; calling at a few stations along the South Transpennine Route, then doing the same in reverse
>https://www.railadvent.co.uk/2023/07/london-north-eastern-railway-runs-trial-train-to-liverpool-street.html...and running-in to Liverpool Street in the small hours of last Friday, as a 'plan B' in case Kings Cross has to be shut for whatever reason (i.e. the ongoing upgrade of the ECML signalling)
Both of these were test runs, so involved swarms of orange hi-viz'd LNER and Hitachi staff checking platform heights and gaps and such.