I called the passenger information number in Immingham and spoke with a live agent…
Foot passengers who do not have a vehicle will not be permitted to
board on either side;
Passengers on push bikes may be permitted to board in the Immingham,
unclear for the Brevik side;
The entry point in Immingham is covered by the same Statutory
Instrument governing airlines, so the UK Border Force will be
alerted from Brevik if they are carrying non-EEA passengers.