For various reasons I for a time held a Bermuda Passport, same as UK but with Bermuda written on as a dependent territory. I occasionally had problems with it not scanning properly, or rather scanning properly but not coming up as a UK passport (ie a normal one ) and getting a short interview with immigration staff.
I also occasionally got it scrutinised by immigration staff and asked to wait a few minutes -I assume they went to check with a supervisor until I asked a guy at Oslo Gardamoen what he was doing . Answer, this can be a boring job so many of us 'collect'passports from around the world and when we see an unusual one we go and make a photo copy for the collection. It also came in handy in getting duty free goods in Europe since although UK is (was) in Europe Bermuda was not so I could get the traditional full duty free allowance as anon Eu person. So its possible this guy had some sort of 'modified' UK passport that caused the problem, even so denying bording is crazy a simple phone call would have probably sorted it.