OK, I dug a bit more. Apparently UK is paying for the fences and checkpoints in France, and does passport control on that side of the Channel, so if the agreement with France is ended, the British border controls would be at the English ferry terminals, not the French terminals. So the Brits probably would end up having to build concentration camps.
This doesn’t necessarily follow Brexit. The UK border controls in Calais have nothing to do with the EU - they’re a bilateral arrangement between Britain and France. All UK-bound travellers have to pass through them, all EU citizens included. This is because the UK has never been part of the Schengen open-border zone. So none of this would need to change after Brexit, unless one of the two parties decides that it should. (In the case of France not,of course, improbable.)