It doesn't take long to offload a pax unless there is a bag in the hold to find. In this case if he has a bag he would be stopped at check-in before checking in the bag or at the gate hw willjust be cancelled.
On a domestic UK flight does it ask for passport details when booking? I don't think it does as it is not a UK government requirement yet like Spain or the US. If so what happens when you put the expired date in?