Going in and out of Luton since late last century and crew nor passengers have ever had to go to the terminal.
Fuel stop on way to IAD, the 1 passenger had to get cleared by immigration in main terminal. Next flight, round trip, delayed as all crew had to go to the main terminal and line up at immigration. Next trip, crew change, inbound crew did t go near immigration. So it is "hit and miss".
Mutt