It is not quite about European/non-European. It mainly is about Schengen/non-Schengen. But UK passport holders do not need visa - unlike say Vietnamese - who are very much in evidence on the KE flight from ICN. And these passengers requiring visa have a considerable longer handling time than non-visa passengers. In theory they should be in another queue - but as most of them cannot or pretend not to be able to read they are clogging all channels.

I think the theory holds. You were coming in from MAN (non-Schengen - the UK has opted out of this particular treaty). So if the plane (for operational reasons) then docks at C (Schengen) you are bused back to A. Now suppose you have to go on to some Schengen destination you need to walk back to C and enter Schengen through the security control at the end of long corridor between B and C. That will require security control (as the C and D gates do not have security control at the gates, but depend on security control (landside) and this control at B/C.

Also there should be passport control (at least starting on December 21st) as otherwise you could say fly to AMS and just walk out there without any official passport control.