Francis, you should be able to take anything that's legal.. can't take guns, explosives narcotics etc.. common sense.
Having read comments by others, I must agree, we take bacon and sausages for a good hearty breakfast, but then eat local produce. I love their sausages but the wife is more fussy about her bacon.. France does not seem to have bacon like we do !
From a practical angle.. you can get excellent quality food there, well worthy of trying.. could save a fair bit of towing weight. fresh milk is available in supermarkets - plastic bottles with a blue top is semi-skimmed.
We too take a bottle of wine for the first night, but then buy-in.
As far as being checked at customs.. (touch-wood) we have never been checked going out, Dover Port nor Eurotunnel and have crossed three times a year for the past seven years.
Coming home, expect to be boarded by UK border control.. situated in France. Generally, they are looking for illegal passengers, not contraband, and usually the check takes only a few minutes.. cupboards, toilet and under the beds. You may also be searched when you arrive at Dover, but seems they know who they are looking for at that point.
PS - French baguettes are far superior to our efforts, but will dry out in two days. Take a baguette storage bag and they last nearly a week.
Richard