France would decide which surviving ships to keep

Would Britain simply refuse to support the least useful French ships?

No; an intact French naval ministry would manage its own fleet and decide which ships to maintain, replace, or retire.

If the whole French Navy continued fighting, French authorities would look after its infrastructure and decide what to do with its ships. New vessels could come from British production lines, as they did for the Dutch, Greeks, and other allied navies in exile. Purchases from the United States would also be possible. Another option would be sending existing French ships to America for repair and reorganization.