In answer to WOODSMOKE there is a definite place, for some uses, for a 12 bore round ball. But as below time and tide have pretty much seen it replaced as a projectile in most shot guns. Although even in the 1980s Eley loaded, for export,a three ball .410" for ape shooting apparently. Three LG letter shot size (36") lead balls I think. I know that they were trying to get rid of the stuff when they had the big re-organisation in the 1980s.
But over the years new inventions like the Brenneke and Forster slug have pretty much seen the single projectile 12 bore round ball load pass into the faded pages of old Eley-Kynoch catalogues and forums such as this. Or been excluded by changes in the law. Thus since the 1960s here in UK the law has said that where 12 bore shot guns are used on deer that any "slug" must be a rifled slug and not a ball.
You are correct in saying "
Surely a solid lead ball can't be classed as 'expanding', can it?" Lead balls although they do flatten out and maybe even expand a little are not classed here in UK as expanding.
in answer to HOWA243 the projectile, the Federal with the blue tip, is indeed expanding. It is no more than a plastic tipped type bullet in a plaswad. Here's a link to the Federal Ammunition webpage for it. The customer reviews tab is worth clicking also:
http://www.federalpremium.com/products/details/shotshell.aspx?id=995
So definitely in the UK would require an expanding projectile or ammunition authority to be possessed either as a loose bullet or loaded cartridge.
In answer to SOUHERN there's very few here in UK that load shot gun cartridges in the usual calibres. First because European cartridges are cheaper than they have ever been, second because the cases here are not true brass bases as in the USA but cheap brass washed steel bases, and third because for wildfowl we now need to use non-lead shot. Either bismuth (and I'm not aware that this can be bought in bags...but may be wrong) or steel.
In fact other than blackpowder loads I've never reloaded shot gun ammunition and certainly have never loaded slug nor possess any. As an interesting fact RIFLED BARREL shot guns of pump action or self-loading would be illegal here in the UK. Thanks to the wretched Mrs Thatcher's 1988 Firearms Act.