Jeans but with some nice shoes, right?
No, I'm not the sweatpants type! But back in America where I lived, there were really only chain restaurants like Fridays or Chillis. Of course it was perfectly acceptable to go in jeans, t-shirt and trainers.
I have some dark jeans and a nice shirt, but no nice shoes. It's a whole 'nother topic, but to me, British casual wear is nicer than your average American casual wear. I feel like I've come over unprepared!