Thanks ILJJ - well I guess we have only ever camped in the UK and having a fire is an integral part of what we do when we are camping. It would probably be different if it's nice, hot weather and probably swimming and other activities would be more the focus. I can totally understand why fires would be an issue, so should now do a new post for general recommendations ;-)
None of the places in either SW or SE France would allow fires or even BBQs. Having seen the consequences of fires down there I'm not surprised.
We camped next to the Lac de Serre Poncon which is sort of Southeast. That allowed BBQs as it was much more green. Still lovely and warm and you can swim in the lake (don't know about in May, though, it might be quite cold).
The sites don't allow fire buckets, either, or any kind of open fire. They're really hot on it ime.
Next May half term we are thinking perhaps to get the ferry down to Bilbao or Santander then drive in either direction - either SW France or Northern Spain. Campfires are a must for us so does anyone have any suggestion?