The work round for AF is primarily connections and those do mean CDG sure enough.
Time wise its about the same from Montparnasse to Nantes on a TGV as it is from Manchester - Euston, however more than a hour longer on cheaper classic TER services.
Its about at that 2.30 limit point on TGV services almost four hours on some TER services
Its a sight cheaper per mile than Avanti especially if you buy InOui !
If I use the train and I have from London I'll change at Lille to be honest
Sorry we have diverged a little