i've been on a couple with Keycamps.
We went to the Vendee coast which is a managable days drive from Calais. But I live in South London so I can be at dover in an hour. We leave early, get a 5am train and arrive in France at about 7am (you have to add an hour as the clocks go forward). Then staright down the toll roads with a few good stops takes 6-7 hours. We go via the tunnel, it is more expensive but so much easier.
You cant guarantee the weather but everytime we have been it has been more sun then cloud. It is a trade off between driving distance and weather.
To save money you can approach the camp site direct and book through them (it is easy via the internet), it will be cheaper but you may get an older style caravan and you cant use the kids clubs as they are provided by the travel agents, although you will have full access to the pools etc.
Most vans are very modern with ensuite bathrooms, some have decking and BBQ's so most nights we just cooked outside.