I don't see much value in the French IFR other than issuing pressure on the EASA. For FAA IR holders it might be a cheap add-on to fly F-reg in France but going this route for current PPL holders doesn't make much sense to me. Maybe there will be an easy path from the French IR to the upcoming EASA FCL.008 but that's a bit of a gamble.
Don't forget that an F-reg airplane in the UK is not a French registered airplane but an EASA registered airplane in the first place and thus subject to EASA IR requirements. Even if their rating is ICAO compliant, you can't use it outside France on F-reg. They would have to introduce a non EASA F-reg variant (similar to micro lights).