Originally Posted by
joejones
The French let the British do pre-clearance on the French side of the Channel Tunnel...
And vice versa. In fact the clearance is done at London St. Pancras and at Paris Gare du Nord in the case of the Paris<->London services.
I just travelled this route and we cleared French immigration (no Customs since it was the EU) at London St. Pancras. It reminded me of clearing Swiss Customs & Immigration a few years ago; the officer didn't say a word to either of us.