You have enough A levels to do anything really at a top uni. If you wanted to do it for self satisfaction then I don't see why not, provided it will not mean you get lower grades in your other subjects. Don't do it if you won't have enough time to get a good grade in it and all your other subjects. I personally don't think you will benefit very much (in your Engineering degree) from taking Economics as anything you will need to know will be taught there anyway, and I can't see there being extremely difficult concepts which will require an A level to understand.