By way of a bit of follow-up it was cheaper and better to take a car with bikes, and park it in France, than to leave the car behind. No idea why. Also, certainly on P&O there are very limited crossings that allow foot passengers.
We did a sort of “park on my drive” and the Hotel Heco in Calais, and it was very safe (gated).
We rode a mixed on/off road route to Wimereux, via lunch in Wissant. The hotel we used https://www.hotel-saint-jean.fr/en/ was very bike friendly and I would certainly recommend. It would have been better if it hadn’t rained stair-rods about a mile from the hotel, and the wind on the way back was 50MPH. Fortunately it was on the tail.
In all it was brilliant, despite the weather, and I would do again (but probably via the tunnel next time).
The ferry *was* cheap, but it turns out that is because P&O are just about the worst employer in the world https://www.bbc.co.uk/articles/ceq3pw038g8o
LR