I thought mandatory handling was illegal in the EU unless there are 2+ handling agents
That sounds nice, and logical, but I should be glad to see it substantiated. Because, if it be true, the below passage from the AIP for EBAW is indeed illegal:
Handling is compulsory for transport of arriving passengers and crew of general aviation and business flights, parked on apron 1 or general aviation terminal (local flights excepted).
For handling, please contact
follow extensive coordinates and contact details for one (1) company.