Thanks a lot for the info... The UK is a special case in Europe - we often joke that nothing is done over there as it is in the rest of the European Union
I did a lot of reading since yesterday, and found out that France will only accept ISO 11784 microchips for pets imported into the country. Unfortunately, those chips, considered "universal" are difficult to read in the US, where the market is dominated by Avid and HomeAgain, whose chips operate at a frequency of 125kHz, against 134.2kHz for European ISO 11784 chips.
The "universal" scanners promised by different brands are apparently not distributed as widely as needed, and most shelters and vets still operate in close association with either Avid or HomeAgain... and their scanners may detect a ISO 11784 chip, but not identify the number and encoded information.
So... I have contacted several microchip companies for more technical info; I may have to end up with two chips on each cat, one that is sure to be read by most US shelters and vets, and one for when I eventually move back home
I wish the European Union and the US would stop fighting over every single detail of every possible subject, and start working towards universal solutions... be it over issues like this one, or commercial regulations, they look like two kids fighting over their shovel in a sandbox