Each time I've been through the tunnel the bikes have always been parked up well away from the cars, therefore loads
of room for a jump seat to be deployed.
Granted it would be a squeeze if unfolded adjacent to car, but surely common sense could be applied not to squat
down if rubbing up against a car. In such circumstances it would revert to standing room only.
BTW, got no chairs in your pub or is it hemorrhoids?
Just open a rear door in one of the nearby cars and you'll probably find a comfy seat. When they see a biker they'll probably shrink back in fear and offer you their flask of tea, maybe with a custard cream biscuit. Remember to leave the car on arrival in Johnny Foreigner land.