Yes - including any carry on bags, and the plane is checked while passengers are dealing with pre-clearance to make sure nothing has been left on board.
You are shown a photo of your checked bag(s), asked to confirm it is yours, and then the usual immigration formalities - resulting in a nice SNN stamp.

Incidentally, last time I was on the LCY-SNN-JFK flight, when boarding back after pre-clearance I asked the first officer what would happen if someone had immigration 'issues' and was told the plane wouldn't wait for them more than an extra 20 mins or so, leaving the passenger in a potentially awkward situation - especially if on the later flight