Bet you're rueing not asking them before boarding
My guess would be that they were citizens of either the Isle of Man or the Channel Islands. As Crown Dependencies, neither are part of the United Kingdom nor the European Union.
Residents of Cown Dependencies are entitled to apply for UK passports as British subjects but they are not British citizens. They are not residents of the EU and do not have any of the automatic rights that EU citizensip bestows (e.g. right to live and work in another EU country). I
presume that as they live outside the EU, there is no requirement for their passports to be standardised in the same way that passports issued by EU countries are.
That's my hypothesis...