What's the real, practical problem here, leaving aside the organisations? I'm assuming that no-one is fundamentally emotionally attached to the actual metal or single-use plastics. So the objections are that alternatives are too expensive, that steel can damage old guns, that they're too expensive, that they don't kill as well as lead, and that they cost too much, right?
By stating that in effect, 700,000 people are going to start buying non-lead cartridges and not just wildfowlers, it's a massive incentive for the companies to develop their paper-cased, biodegradable, non-lead cartridges, invest in the testing, production, distribution. There will be competition between manufacturers, prices may drop, and they'll develop an offering ready to export to Europe assuming other countries follow suite. If you do a lot of shooting, you'll probably buy a new gun. You can still use your old one with bismuth from time to time if you want to. Admittedly it's probably worthless, but second hand shotguns are already worth very little outside the very top end of the market. I guess it's no different to in that respect to when engines switched to lead-free petrol, and now to electric.
So it will take a bit of adapting. Cartridges aren't complicated things though. The manufacturers should be able to sort this in 5 years.