Reflecting on the last few years, the constraints for a Fridays' journey are roughly:
- Easy to get to from the South-East of England, preferably without reliance on a single train or ferry line.
- Enough largish towns that there are enough places for everyone to stay, preferably with a good spread from campsites to 4* hotels.
- Towns close enough together that we can leave after breakfast and arrive in time to look around - which means a maximum distance of 50-odd miles on the flat, or a bit less with hills.
- Countryside that is reasonably picturesque and towns that are reasonably interesting.
- Bail-out options for injuries, malfunctions, hangovers, tiredness or people who just fancy doing something else for a day.
If you can lay on a bus and a van to carry people and bikes and particularly luggage you can relax some of the constraints - in particularly the distance constraint - if the journey is interesting or meaningful enough (see Bordeaux and LonJoG).
Sadly, I suspect Britanny is just too difficult for a typical Fridays-sized group without that vehicle support. The only way I can see to make it happen would be to start and end from St-Malo, because Portsmouth is reasonably approachable - but you're still constrained by basically one ferry per day and one route. Either that, or have a much smaller tour (?8 people) - which is exclusionary.
I like the idea of a history-themed tour - but you'd need to leave time to look around places (and heretically I think one battlefield or memorial is much like the next). One other vague thought I had was a border-themed tour - roughly follow, rather than cross, land borders. In the low countries you could combine that idea with a visit to Baarle-Hartog or the Vennbahn (look them up - relics of history). On the way to Dunkirk ferry I was idly musing that Dunkirk town to De Panne would be a good short morning - lunch by the seaside to start the holiday, followed by a trip to somewhere like Bruges in the afternoon. You could also pick themes like breweries or belfries with carillons or historical churches (though I recognise some of those are minority interests).