The ban on lead ammunition is part of REACH legislation which covers the use of dangerous chemicals.
Penalties are pretty severe including unlimited fines and jail time, and includes directors etc who knowingly allow the use of dangerous chemicals and these chemicals getting into wider environment.
Enforcement of REACH is in the hands of the Environment Agency, the Health and Safety Executive and their equivalents in the devolved nations.
Lead based ammo will fade out as the deadline on April 1st 2029 comes about. It will just like the fade out of IMR 4831, it could no longer be imported and stocks just got used up.
Of course there will be individuals who will stock pile lead ammunition and cartridges and will continue to use them. But increasingly they will find it very difficult to find places that will allow them to be used. In game shooting the onus will fall on shoot captains and with commercial shoots, the directors. Some may turn a blind eye.
As regards target shooting and continued use of lead bullets.
1) it will only be target bullets that can be offered for sale and use.
2) approved ranges - these will have to have facilities to capture and allow recovery of lead bullets. The approval will be from the Environment Agency, SEPA etc and the range operator will need to record how much lead is going in, and when lead recovery happens. The Agency will inspect the range, and the operating procedures prior to granting the derogation. There will be a nominated person within the club or range operating organisation who will be responsible for ensuring the range is compliant and will act as the designated inmate if none compliant. Note comments above re penalties.
Exactly what the Agencies will require in terms of range design etc remains to be seen.
Again it will be down to individual clubs and organisations to choose whether to comply or disregard the law and risk the penalties.
As for shotguns - unless you are an Olympic standard athlete with a derogation to use lead you won’t be able to buy lead cartridges.
28 bore and 410’s are difficult to find steel options in the UK at the moment, although they do exist overseas. You can buy bismuth shells for both in the UK.
Given the market demand there is an opportunity for a market niche for steel and non-lead options in 28 and 410.
If you are shooting old obsolete guns, then you are probably already rolling your own cartridges. I suspect those of us shooting 2” and 2 1/2” 410s will be loading our own with Bismuth. Mind cost of 410 cartridges are already eye watering compared to 12s.