Black pudding is part of the full English in Yorkshire, but not more southerly than that, as far as I know. It's not surprising to find it in London- you can find anything edible in London, including a restaurant that specialises in cooking animals whole and letting nothing go to waste. I forget the name, it's one of those trendy expensive ones so I've never been.

Where I live in the South, the full English is french toast, baked beans, hash browns, fried mushrooms, grilled tomatoes, eggs, bacon, sausage and spam (sorry, couldn't resist). I sincerely believe that a majority of English people would not touch their breakfast if it had black pudding in it.

Also, I'd think the middle classes would be more likely to eat offal, just to show they're superior to the plebs. But I might be wrong.