London School of Economics (BSc Economics):
AAA, one of which should be Maths.
I also have read the Students Room and surprisingly found out that they admire people with AS in Phychology.
University of Oxford (Economics and Management):
AAA, Maths is strongly recommended
Warwick University (Economics):
AAB + AS level B (or C as a fourth A level) or A level AABB for successful applicants taking 4 A levels inc. Maths and Further Maths. Applicants are encouraged to avoid combinations of subjects with significantly overlapping curricula such as Economics and Business Studies.
University of Southampton (BSc Economics):
A levels: AAB
AS level: mathematics at grade C required, if not offered at A level
University College London (BSc Economics):
AAA including Mathematics
University of St Andrews (Economics):
I also wrote to LSE a letter asking for further information on entrance requirements.