Here's a few thoughts.
A rockhopper will be fine. Lock out the front suspension. Semi-slick tyres are also fine, no need to change them.
Personally I use Vaude waterproof panniers. Waterproof panniers do tend to be pricey, though. Some get by OK with their gear inside plastic bags inside non-waterproof panniers. You'll need to fit a rack.
Yes, just lock the bike to something convenient and close by when you're asleep. If the lock is moderately substantial it will be difficult to steal the bike silently.
No, don't try to convert the bike to drops unless you are prepared to spend some money. Quite apart from the difficulty of setting it up to get a decent riding position on a bike designed for flat bars, you'd need to replace the brake levers and gear shifters.
Saddles are a pretty personal choice. I suggest you ask your LBS if you can try before you buy.
As far as I know, you won't be able to charge your phone direct from a dynamo without a gadget like an e-werk. Not cheap. Others might know better. Take a power monkey, would be my advice.
It's worth knowing how to fix a chain (and to carry a few spare links) and replace and/or tension spokes. That would add a chain tool and a spoke key to your list of tools. You should also carry some tyre levers and a set of allen keys.
Hitch-hiking with a bike is pretty difficult. I'd just ride, if I were you, and put the bike on a train if required. And on that subject almost all the UK train companies allow bikes, usually subject to booking in advance, so there's nothing to stop you travelling by train from Manchester to the South Coast. But there's nothing to stop you riding it, either.
No special safety tips for France except keep right. In general, you'll find French drivers more considerate of cyclists than English ones.
Don't worry about planning the route too carefully. There are lots of municipal campsites and international hostels that are of good quality. But if you are riding between September and November, more and more campsites will be closing as you go along. Best to check in advance. If it were me, I'd go April-June rather than September- November.
You don't really need special clothing for touring, especially if you're only planning on 30-40 miles per day. Some baggy shorts with padded undershorts would be a decent investment, though.
A budget of €20 per day will be tight, even if you camp virtually every night.