It's already being done.
Pressure will be put on the foreign manufacturers to comply with the AT system (although not legally required). Obviously HW do it through Hull Cartridge (who are members of AMTA).
Whether this falls foul of EU laws I don't know but it's certainly restrictive practice. There is a threat to the AA and Daystate offerings from foreign manufacturers. Who would shoot with a 10.5 FPE EV2 against a Walther shooting at 11.5 FPE ?
The last time U.K. manufacturers had anything to do with the law we got the 12 FPE limit. Now we apparently are not responsible enough to ensure our rifles don't go above the 12 FPE limit.