Thank you both, I’ll order these maps 
I would suggest that the 1:25,000 are good they have too much detail for cycling (you'll loads of them!) - the 1:100,000 is quite adequate (equates pretty much to the old 1/2" to the mile that was the cycling map of choice in days or yore!)
This is good advice. I would add that the atlas of the Voies Vertes et véloroutes needs to be used with a good deal of caution. I have it and was planning to follow the Scandibérique near me. The routes shown on the latest IGN 1:100 000 differ from the ones in the atlas. On the ground I have found signs that follow the IGN version. There is quite a distance between the two possibilities so I am not sure the atlas version even exists. This may be a one off case but I doubt it. Be warned!
The latest Michelin départementale maps 1:150 000 (there is probably a regional one also covering the whole of Brittany) are also quite good and show a lot of the voies vertes and GR routes. Handy at the planning stage.
A good way to see what is available is simply to google the département (Morbihan, Finisterre etc) + véloroutes. You need to do this at a départementale level as the provision and politics are départementale, not régionale. It helps to read french but I suspect that english translation would be readiliy available throughout most of Britanny. Don't forget that Loire Atlantique is also technically breton even if it's not counted in the région.
The IGN 1;25 000 (Bleu) may have caught up but there was a stage about 10 years ago when the road mapping on paper was a long way behind the latest developments on the ground (particularly footpaths IIRC). It came as a bit of a surprise since it's the official goverment cartographic agency! I think they may have put more time into the web services!
