How effective is a Smoke Control Area at reducing particulates? (UK)

Particulate is a very local issue, so it really, really depends on your local conditions. Particularly your ratio of greenery to urbanity. Downtown London, oh, you betcha - they've had smoke controls for 120 years. The underground used to be pulled by steam engines, if you can imagine!

But if you're in a rural spot where the trees outnumber the people 100,000 to 1, the particulates get gulped up by nature pretty fast, and little real gain will occur from burdensome government regulations. At that point, it's "regulation by spite" (since Londoners can't, neither can you in the Orkneys) or "regulation to harmonize rules" (you can't since Prague and Berlin can't), or "environmental theater" (look at how clean we are on things that don't matter).

Another factor is the burden of enforcement to the users. For instance if it will disproportionately impact the very poor, or historic operations (canal boats or heritage railways, then the social impact needs to be considered in balance.

Carbon emissions, of course, are a different kettle of fish; and not a local matter at all. I don't know how your jurisdiction views "new carbon" (wood) vs "old carbon" (coal).