Thanks Hoodie
The guidance is dated 2018 so it is possible the rules were relaxed - I am aware of a nurse on a Slav passport on an interhospital flight who was seriously delayed a few years ago, and my correspondence with the deputy director of Border Force, again some years ago, was clear. However all section 9 refers to is visas - it does not specifically mention non EU EEA passports. I am not an immigration expert so I suggest anyone in this situation seeks specific advice.
You could get a situation where there is no stamp recording immigration or emmigration. Border Force do check for these at immigration - my partner was questioned at LHR. Does Border Force enter the GAR data on their computer??? And what if you cancel the flight - you could be accused of being an overstay. I am sure they have an answer, but best to ask and not rely on PPRuNe