Upvote:4
Border control for both countries happen together before the tunnel no matter which direction you're traveling in. (Source). These happens by a special agreement between the UK and France ("juxtaposed controls").
The UK does not use exit stamps for anyone -- but you can expect the French guards on the British side to stamp your passport with for a Schengen entry.
Anecdotal reports suggest that the French sometimes don't give an entry stamp. If that happens, try to politely insist on getting one; it may create trouble for you when you leave the Schengen area if you can't document when you entered.