That's definitely not the case in the UK. For example, the guidelines for applying for Economics at Cambridge state:
"The Faculty does not have standard requirements for subjects other than Maths: in particular A level Economics, though useful, is not a prerequisite."
(FWIW I did mathematics as an undergrad but I considered CS and spoke to CS faculty at various university open days, from what I remember they all considered maths to be the only essential A level for admission.)
"The Faculty does not have standard requirements for subjects other than Maths: in particular A level Economics, though useful, is not a prerequisite."
http://www.econ.cam.ac.uk/ba/guide.pdf
(FWIW I did mathematics as an undergrad but I considered CS and spoke to CS faculty at various university open days, from what I remember they all considered maths to be the only essential A level for admission.)