Br, I actually cannot tell the difference between Scottish people and Irish, and most of the time I can't tell the difference between English and the other two. I haven't met enough Welsh people to comment on that though.
That seems to be the result of bad choices when it comes to diet. Surely, they're not forced to eat more fast food and junk food than they should. You can buy healthy food at low prices. I'm not sure that current conventional wisdom is correct when it comes to the causes of obesity.
From what I read, there isn't too much variation in appearances, which makes sense. British and Irish people cluster in close proximity to each other since they're both northwestern Europeans. You likely find darker-featured natives in the western fringes of the Isles, though. I'm not sure where the most attractive folks live.