Wonderful. What will gardeners and potting mix companies use as a substitute for peat?

I don't know, but there are potential alternatives like bark and other forest products. I am not sure about England's peat bogs, I assume they are fossil ones (not alive and being added to). In Canada there is Sphagnum peat which is derived from a small plant that grows on the top of the peat deposit and slowly adds to it. Theoretically sphagnum can be harvested sustainably although it grows pretty darn slowly. UK may have to import more products like that if they don't have domestic sources.

The UK has been advocating peat-free compost ("compost" being the UK terminology for a container or potting mix) for years. Typically they are comprised of ingredients like wood fiber, composted bark, coir and sometimes composted yard waste. They can also include things like composted manures, grit, perlite, vermiculite, sheep wool and composted bracken fern.

That's about as British as you can get, sheep wool and bracken fern! Love it.

@tox, I find the terminology of peat and bogs and all that confusing, but the 'Sphagnum' / sphagnum moss in Canada is (as I understand it) the main component - alive or accumulated - of peat bogs in most places, at least northerly places. For some reason in Canada the term is just used more, but to some degree peat and sphagnum are more or less the same thing, just the preferred term used for the live plant.

I think the sustainability argument in Canada is pretty simple - they just harvest so little of it that it barely makes a dent, and they're not permanently draining the bogs to get it. (Also a lot of the bogs are so inaccessible they'll never be harvested). I'd argue it's still not great to use it in North America for general purposes -local alternatives from waste products better - but for seedlings and the like, perhaps the advantages outweigh the negatives. (Canada probably partly got lucky that burning peat never became a thing...wood and hydro were more plentiful) Anyway a different environmental issue than in Britain.

Whether it's 'fossil' or not - I don't know the situation in Britain. In Moscow region, the issue has been draining of enormous bogs - peat from which was used as fuel for a couple powerplants into the 70s, I think. Those dry peat bogs have a tendency to burn, badly, once every ten years or so (the Gorky Park novel has a bit on this) - and because underground are almost impossible to put out (they say some of them never stop burning). I gather those are 'fossil' in the sense that the drainage means they don't replenish any more.

I'm guessing the British situation is a combination - some drainage of older bogs, some that were overharvested for centuries (partly for fuel), and they grow back far too slowly.

Anyway, sorry for the digression.

This thread led me down the merry internet. I see Ireland's phasing out peat as fuel (commercial anyway) very quickly.

https://www.bbc.com/future/article/20201203-peat-the-decline-of-the-worlds-dirtiest-fuel

And I was surprised to see that Finland still burns a fair amount of peat for electricity. Moscow still has a single power station that burns peat, although not very much (~10% of a 1500mw station, the rest mostly gas, so I'm guessing it's an older and smaller unit - surprised it hasn't been retired or converted).

I'm guessing that getting rid of peat as a fuel is a far more significant step than peat for gardening, but don't have any sense of the scale of how much is used for that (one article referred to 2m cubic metres in 2019, but doesn't mean much to me).