If Oxford Road could be dropped by TPE (ie doors not open) what becomes the ruling length constraint instead, ignoring the current length of the rolling stock?
Manchester Airport, arguably. That's 196m, i.e. enough for 8×23m, however even now with 4 platforms, platform sharing is inherently required in most timetables. So that would require an extension, which would be quite difficult to do.
Piccadilly's low numbered platforms is a similar situation, albeit they are a little longer (the shortest is 3 at 237m) so you could probably 'get away' with it if you modified the track circuits to allow platform sharing with a long train at the buffers. Obviously an extension is completely impracticable.
Huddersfield would be an issue until it's remodelled as part of the TRU scheme. It might be another 10 years from now until that's done.
Scarborough only has 1 long platform (no 1), which is also the only one to have access to the excursion sidings, turntable and TMD (so can't always be used, particularly during the summer when the Scarborough Spa Express runs). The next longest is platform 2, which at 183m wouldn't quite fit a 8×23m train. So that could be an issue.
Newcastle has a decent number of platforms, including bays, but only the through platforms would be long enough for 8×23m, so again, that's a bit of a nuisance if you are sending 2tph there.
Various intermediate stations aren't 8×23m long, but as long as the majority of the train fits on the platform, SDO is a short to medium term 'sticking plaster'.
So basically - 8×23m isn't totally impossible, but it is right at the limit of what is possible and it would require a lot of compromises (likely including in the timetable) to make it work.