You are having all the same dicussions as we had 30 years ago, when we where forced to use steel

.
Nowadays we got steel cartriges that do the job, but i dont know if you can get them in UK.
The ones shown in this tread are unkown to me.
First off always use larger lager pellets than when using lead.
For shooting phesants we used 2,75-2,5mm shot size lead in the good old days. When using steel 3mm is the norm, smaller simply pass right through them.
Same reason we used 3,5mm lead pellets for deer in old days, now we use 4mm steel. The 3,5mm steel pellets often simply pass right through and the deer will run a distance before it dies.
A lot of shots on big hunts hurts: Think thats the reason 20Ga has become so popular here.
12Ga leadshot with 36gram load, has less recoil than a 20Ga steel cartrige 24gram load.
If shooting a lot of 12Ga steel then a nice blue shoulder and well establised headace if for certain.
However if it was up to me I would still like to go back to lead.
Its more pleasant to shoot lead, and standard Eley Grand Prix lead cartrige kills about as well as the best (and 5 times more expensive) steel ones. Thats a big deal when we use them for shooting deer.