Some plug in points are same as ours but many are still on the two pin variety. You need to take an adaptor available in any decent caravan & camping shop. Its just a short length of cable with the 2 pin plug in on one end and the standard uk socket on the other.
Some mainland european sites are also reverse polarity. You need a polarity tester and adaptor to deal with this.
Just take your normal EHU & the 2 pin adaptor as stated, you may or may not need it in France, depends on the site. I wouldn't worry about reverse polarity as long as your electrical gear is modern, it will be to the same Euro standard as anywhere else in the EU.