TBH. I think they are bigger in the US than they are in the UK nowadays.
I don't live in either country so I couldn't tell taking into account all parameters. In per capita sales, they're still bigger in the UK. But what about cultural relevancy ?
Honestly 1D's biggest achievement, and now especially.
Indeed. When you look at past boy-bands and girl-bands 4 years after they began, they've struggled to maitain relevancy. I think 1D might be the boy-band with the most longevity since the early Beatles.