LSE Maths and Econs offer is AAB, which is easy to meet. However, beware that they reject many people outright, i.e., LSE doesn't even give them an offer.
Warwick Maths and Econs: I didn't have econs but I got in (but I have 2AEAs). Warwick's offer for this course tends to follow the maths one, which is AAB and Merit in AEA/STEP. It is arguably harder to meet the Warwick's offer with the AEA paper in place.
If you have made up your mind, I suggest that you also ask at the economics forum. If you want to have maths and econs knowledge, another possible approach would be to do BSc Maths and MSc Econs, which would arguably give you a strong foundation in both, while putting two top universities on your CV, possibly with contrasting "feel". "Edders" did BSc Physics at Imperial and is going to start MPhil Econs at Oxford this year.
Again you have to consider for yourself.