You seem to be quite hung up on not having been on a motorway. I would have thought that a motorway would be of little value to a driving assessment. No kerbs, parked cars, opposite traffic flow, pedestrians, less traffic signs and road markings… most drivers can cope with a straight road, so it hardly gives “the whole picture”. You aren’t even taken on a motorway for your test. I would see it as a waste of time and fuel to see if a driver could go a junction down the motorway and back.
The assessor could easily make a decision on your driving and attitude based on what you have told us. It sounds like a reasonable assessment to me. Maybe for your next one, you could tell them what you expect so you are not so disappointed…