Merlin,
That's a very interesting idea.
Thanks
The French had slightly more than 40 boats in the Med when they surrendered and all but 2 or 3 stayed with Vichy. IIRC, Britain actually had to sink one during the Syrian campaign.
Having more French subs "come over" to Britain in 1940 could be helpful if the French were willing to operate under British command like the various Norwegian and Dutch naval assets did in the OTL.
Of course, whether the RN, or anybody else, would want to take the time to learn how to operate the French boats, whether the weapons aboard those boats could be refitted to to WAllie standards, or whether the boats would be seen as useful are all important questions.
Still, when you consider that Britain was forced to redeploy all it's submarines in the Far East to the Med and Red Seas when Italy joined the war, having the prospect of extra hulls at hand could be attractive.
Bill
Regarding, converting them the W. Allies standards, when needs must - doesn't that just mean re-labelling 'french (language) instruments, changing the wireless equipement, and re-loading torpedoes (though perhaps the electrical system may be on a different voltage)?
The French did have subs in the Pacific - seem to remember the uniquely French one with an 8" gun!! Perhaps, the RAN could comandeer them.
Or at least in an ATL, it could be plausible.
As regards - buying them elsewhere, I think it would more likely be the case of:
'If only we had some subs, we could really do some danger to the those Jap' ships'.
'What, about those, we have over in - Freemantle(?), I know they are French - but they won't be using them'!
That is , a coincidence of need and availability.