That's a good point. It may be much less expensive for you to buy yourself in France as things are generally cheaper on the continent than in Britain. It's that strong pound you guys got.
I only use two rear panniers, and I've never had a problem. If it all fits in 2, why use 4?
What do you mean by spare equipment? Do you mean spare bike parts? If so, then I recommend putting new Schwalbe Marathon Plus tires on your bikes before you leave. This way, you won't need to bring spare tires, or spare tubes as you won't be getting any flats if you have the S. Marathon PLUS.