There are some sites that use standard "UK caravan site" style plugs, Parc Verger included, but what you should carry for the other sites is a continental adapter so you have a continental-style plug on one end with a UK caravan site style plug on the other. Then you will have no difficulty plugging in your normal lead to this. You can buy them in many caravan dealers, and we can buy them in our local hardware store here in France.
On reverse polarity, do carry a plug that will check this - it will tell you if there is any potential problem and what it is. As has been said above, reverse polarity can leave your electrical equipment live - not a good idea!
Best bet - find an experienced english-speaking motorhomer (or go to an english run site) when you get to the continent and ask! Don't be afraid, we all love to help.
Bob