5 months, sounds fantastic.
Try the GB auto clubs. There will be at least one camping and caravaning club in the UK. They should also have something useful for you.
I used to carry the ADAC guide with me, but it comes in two very large books giving you lots of info you don't need (unless you are very fussy campers, perhaps...) and it only shows a selection of campgrounds.
I don't think there is a list or map showing all campsites, even in one country.
You may traverse large areas of limited touristic interest occasionally and there are no campgrounds at all. Don't hesitate to ask a farmer if you can camp in a paddock.
In France look out for signs "Camping à la ferme". Often good value and quiet.
Have fun!
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Salut from Southern France, the bikers' paradise,
Peter.
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