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England, Scotland et al don’t count as different countries because their brevets are all validated by AUK. The four BRM or LRM brevets need to be homologated via different national organisations.
The easiest/ least-carbon-intensive ISR might be a BRM SR via AUK, Audax Ireland, France and Netherlands. The gold standard is 4 x 1200+ brevets, each done on different continents. I’ve got one of those, back when I travelled more for work. There is no time limit to complete the four rides and you can apply with historical rides.
Yes that's the Ultra Randonneur patch... I've got 7 left to do for that one : /
There's also the international SR which might be a fun one, you do your 200/300/400/600 in different countries. I'd like to try that only using ferries...