The change in the REACH law hasn’t made 243’s illegal. A 243 that can’t make 1,700 ft lbs with an 80gn copper bullet, almost certainly won’t it with an 80gn lead bullet either. So your 243 has always been illegal for use on big deer.
In Europe, the minimum for big game is 2,000 joules at 100m, hence the 243 is not a popular hunting cartridge in Europe, and hence limited options in European manufactured ammo. Many of the european manufacturers now load short barreled ammo for likes of the 308, given widespread use of moderators. These uses faster burning powders to give faster acceleration of the bullet down the barrel. Whether this approach works in the 243, I haven’t a clue. But laws of physics with the smaller base area of the 243 compared to the .308, and hence less force for acceleration for the same pressure probably work against the 243.