United Kingdom

I think it would cover all four nations (probably including the Republic of Ireland, but that should await confirmation from someone who know that country better).

Chicken gravy on Yorkshire pudding? Yes, you can, but you don't need to bother. Chicken has stuffing as its special accompaniment (plus its crispy skin). Pork has crackling (and in my childhood home, 'dressing', ie stuffing cooked outside the joint). Lamb has mint sauce (or jelly for those with a sweeter tooth), and it's beef that needs Yorkshire pudding. And beef gravy probably does have a little extra taste that complements it (I'd guess lamb gravy would work quite well with it too).

Yorkshire pudding is not needed for chicken. If you need to go beyond the stuffing, then it's bread sauce.