In reply to doz:
To use an old cliché, you've got a match of two halves. It's probably more of a cycling challenge to get to the Channel Ports in the UK than it is to do the other half in France. As has been said, any of the western French ports (Le Harve through to Roscoff) are closer to Toulouse, but they're still 1000km away which is a very steady 50 hrs of cycling, gently spread over 10 days would be very enjoyable. But each day you cycle, you will get more 'experienced' and it's very possible that if you start gently enough and increase the hours each days slowly, you could cut several days off that. Halving it would be a big ask.
Get a Michelin Road atlas with the original 1:100,000 scale mapping and simply trace the straightest 'back roads' route and follow that. Any that look like they might be rat-runs or otherwise busy, you can check on Google Street view. Once you've planned your route, cut out the relevant pages and off you go.
Getting to the English Channel is either quite convoluted via Sustrans routes (including Routes 6, 7, etc) or shorter, quicker but much busier. I would suggest it's worth investigating either Bike Express as has been suggested (but now coming to the end of the season) or the trains from Edinburgh/Glasgow to Birmingham and either Portsmouth or Plymouth. It's a pain in the arse but possible with a bike given enough planning.
I've always camped in France and usually there's a municipal campsite 'there' at the end of every day. But in the north, they'll start to close by the start of the school year. I've never used them, but I've heard good things about the "Warm Showers' organisation.