Exactly growstuff. If anyone has ever been to Calais recently and stayed over for a day or two- s/he would know that 1000s of special force police, armed to the teeth, are permanently stationned in Calais- staying in local Hôtels- from all over France. They patrol night and day, non stop- armed, with dogs, and they are not your average copper, but CRS- highly trained and tough guys. There are layers upon layers of very high fences, dug deep into the ground, angled, with electric currents and several rows of razor wire ... then Border c hecks by both the French and the British, with specialised guards, dogs and equipment- checking cars, vans, lorries, etc, to stop immigrants crossing.
All that will go if we leave with No Deal - immigrants from all over the world know this - and are waiting for when it will be much much easier.