The post you are reporting:
NM: It all started with the Treaty of Canterbury, which juxtaposed most controls for the Channel Fixed Link. Once post-crossing, 'free exit' was seen as a USP for Le Shuttle, the ferry routes wanted it too. What I can't recall is the original motivation; possibly the perceived impracticality of returning refuseniks on Eurostar, possibly the theoretical reduction in immigration staff (and land-footprint) if checks are performed, trickle-fed, pre-crossing instead of pulse-fed, post-crossing.
Personally, I wonder if the advantages for governments are marginal, although they are significant for carriers and port/terminal operators. I suppose that with ferries and shuttles, there's no doubting from which country any clandestines on board last came. However, note that carriers were (are?) fined per clandestine found post-crossing, which meant that P&O and chums did (do?) their own searches of vehicles waiting to board - in other words, there's still a pinch-point that can cause outbound queues when passenger numbers swamp the available controls.
As for getting out of juxtaposition, a close read of Le Touquet will reveal that it can be done virtually overnight and doesn't require a 2-year notice period.
