Hi again Alan,
I would not recomend the site at Bernay, we stayed there 3 years ago, and for a municipal site, it was expensive.
If you have a caravan, the pitches are small and hard to get parked up on. Being located on the top of a steep hill, it's not cycling friendly if you have bykes, and another downside, it's a long walk into the town.
Re a stopover before going home, we use the St - Claire site at Neufchatel en Bray on the A28, its a small private site with good access and it's only 500 meters to the supermarket. The cross channel terminal is under 2 hours drive away.
As you are in the height of the holiday season, I would stay clear of the site at Guines, it's overpriced, and will be full of transient campers. The weekend you are travelling is a 'Big Holiday' in France, where ever you decide to stop, either pre book, or get to your chosen site around noon. They fill up very quickly from Thursday and Friday that weekend.
An alternative site is the municipal at Le Treport, again a very busy site, but within walking distance into the town and ideal for the 'last stop' before coming home!