Not just in the UK
Given that the UK considers its libel laws to govern the planet (hence out roaring trade in libel tourism), then the UK could (and probably will) consider Twitter and any other service globally liable.
The only exception would be where the foreign service blocks the UK. If any person the UK viewed foo.bar.to, then that has been "published" in the UK and foo.bar.to is now liable. Despite not having staff, offices or operations in the UK
At least, that's how it seems to work in dead-tree media.