Take as few tents as possible - Campsites in Europe generally charge per person, per vehicle and per tent. Since you can't do much about the first two, the only way to save money is to bring a single tent that's big enough for everybody to fit into. The downside of this is the possibility that your big tent won't fit in the plot, but most sites seem to provide a pretty big space for you.
Take some kind of mattress - I've always slept directly on the ground in the past without problem, but the campsites in Europe can have much harder ground than the sites in blighty, with little or no grass, leading to some not very restful nights.
Spanish people stay up very late and have exceedingly loud TVs - I guess I already knew they stay up late, but hadn't considered the potential problems when staying in a camp site. Most of Spain takes the whole of August off and some of them head off to camp for a month, along with their tellies. They are then involved in an arms war in an attempt to drown out the sound of other campers' TVs, leading to yet more lack of sleep. In the site we were in, we also right next to the train line into Barcelona... Loudest... campsite... ever...
People still camp - I was under the impression that there were only a few foolish people who still bother to use tents when on holiday but we found quite a few camp sites that were full when we arrived.
You can camp for free in France - As well as the standard service stations on major French roads, there are plenty of aires that are just a bit of land with toilet facilities and running water. We didn't use them ourselves and they are probably only for hardcore campers who are happy to do without modern facilities like swimming pools, shops and showers, but it's good to know they are available if necessary.
Driving is more fun than flying - OK, a 3 hour flight may be quicker and cheaper than a 3 day drive but it doesn't give you any kind of idea of how far you travelled or let you visit any intervening places.
GPS would be useful - A bog standard map is fine when travelling on the major roads but when you get into a city things can get a bit tricky. If you've got a detailed map of each city you'll visit you'll probably be OK, otherwise GPS will probably make your life easier. I can't verify this since we didn't go for the GPS option.
The UK should sign up for the Euro - It's surely inevitable so why not go ahead and do it? Life would be so much easier, no more carrying around two currencies, no more trying to work out how much something really costs. So we may lose some sovereignty, so what? Get over it.
Wine in Spain is very cheap - 8 euros for 10 litres...