Pre-pandemic, Southampton served about 15,000 passengers annually to Nantes (plus roughly another 10,000 each to Rennes and La Rochelle) as part of a pretty reasonably-sized flybe network to regional France.
SInce Ryanair have already launched Bournemouth to EGC and CCS, as someone on here has already said it looks very much like they're parking their tanks on Southampton's lawn.
Assuming there's still a similar 'local' market demand from south of England to these parts of France, if Ryan are putting about 13,000 seats into the Bournemouth-NTE market then logically it looks like this ought to work.