Passport after departure lounge
 

I was booked on a flight to depart Luton to travel to France.
The flight was delayed by over 5 hours and I decided to abandon the trip.

To exit the airport security insisted I be escorted by the airline handling agent. The handler took me to the (post immigration) baggage retrieval hall and insisted I had to then walk with them to the pre immigration hall. Refusal to accompany the handling agent met with threats of arrest by a police officer.

As Luton has (I believe) full separation of arriving and departing passengers, why is it necessary to show a passport to exit the airport when I hadn't left the UK or passed a UK border point ? Note - there is no border control to exit the UK at Luton. I was thinking to prove a point I could book a flight on my Android from Luton to Edinburgh later the same day, just to show there was no statutory immigration requirement (or a loophole existed) but decided not to waste time+money.

Apologies if this seems like a rant but I'm mystified as to where all this immigration requirement comes from. Anyone knowledgable out there ?

If you were in a controlled area available to foreigners then you might be considered "uncleared'

I don't think Luton in the UK has the possibility of transfers - I believe all arriving passengers have to first clear immigration / customs, and then go through security again to reach the departures area.

Anyone in the departures area has presumably just come from landside and therefore either has already entered the UK, meaning everyone (I think) should be "clean".