Hi Nick
My wife and I and our two small kids in 1980 took our Sunbird 32 junk from Germany to the south of France via German and French rivers and canals, popping into the Med via the Rhone and Petit Rhone around Agde. You can also exit near Sete.
We did the reverse trip some years later, that time via the Canal du Midi then up the Brittany coast to cross the Channel to Plymouth. In a shallow draft boat like Otter you could cut the sometimes difficult corner of NW France, entering a canal system at I think Nantes and exiting via the Durance, then cross the Channel via the Channel Islands.
Whichever route you take you need to check depths and air heights.
This site is a good place to start research, and a good reference book is
Through the French Canals. If you choose to go up the Rhone, do it in the autumn and if there's been lots of summer rain or if your engine is a bit dubious, expect to need a tow. The Midi can be shallow in the summer but that wouldn't bother
Otter. When we went that way the Midi was in drought but we got behind a 5 foot draft French boat (ours was four) and followed her to Bordeaux.
We've a couple of members with a barge in France - sorry, can't remember who - but if they read this I'm sure they'll be in touch. As far as I know there are no canals linking Spain to the French system.
( Don't listen to Annie - a cruise through the French Canals is one of life's 'must do' experiences.)