All the shops I seem to go in only go up to a 40" waist. I have a large butt so even though some jeans are really loose around the waist, they tend to be kind of tight on the butt and I don't like it. Do you know any shops in the UK that go bigger than a 40" waist?

Gap goes up to 42 on their UK site and their jeans run large.

Old Navy comes to mind -- they're a bit cheaper than Gap and go up into the 40s for waist with a 30 length. In my experience their jeans tend to fit a bit looser too. You'd have to order them online but I'm pretty sure they ship internationally

I get mine from Next. They go up to a 44in waist, and they look great too. They also have short lengths which are helpful for shrimps like me.

I order mine from Amazon - usually Levi's 559 or Southpole because I like the way they fit.