This has nothing to do with Brexit and claiming it's a fact that it does is just deranged, Conor. The equivalent process would be occurring if Brexit never happened, as you've acknowledged. In your subsequent comment, you've said that the resulting recommendation is less draconian than if we had been inside the EU, demonstrating that even you don't think that Brexit is either the cause of this or an aggravating factor making it worse. To the extent that Brexit has made any difference to this, it has improved the situation, according to your analysis.
There is no necessity to ban lead ammunition in the UK at all, and the trade decision could always have been made that the UK could have managed acceptably with domestic producers. I am not sure what the balance of imports/exports of shotgun cartridges in the UK is, but I would think it very likely that imports exceed exports. As i said, Brexit is a red herring.
The HSE review came about because of anti-shooting activists, quite likely encouraged by BASC's inexplicable decision to promote their bogus pseudo-scientific claims.