physics vs economics A-level?

If you want to take econ at uni, I would say take it at A-Level too; if you're interested in the subject, you're likely to score a good grade at the end of the A-Level. Also, you should be interested in econ anyway, in which case, why would you not take it for an A-Level?

Where do you want to apply? In some cases, taking double maths and econ may be insufficient. Would you consider taking all 4 to A2?

In what universities will this be insufficient? I may consider taking all four but I'm not sure if psychology will benefit me.

LSE takes FM as a 4th subject only, I believe

As of 2019 entry they no longer do that and accept maths, further maths and another essay based subject for economics bsc admissions.

So I can take double maths and economics at A2?

Yes that’s absolutely fine, and as of 2019 entry most unis accept it, however with people that apply to lse also applying to oxbridge, its more competitive, so people tend to have 4 if not even 5 a levels up to A2. But your 3 are still as good. Good luck.

Economics A-Level hasn't got much to do with Economics at Degree level. Many people studying Economics will never have studied it before, so there's little use taking the A-Level, which is more theory based than the Degree.

However, bear in mind that Physics is definitely harder than Economics at A-Level.