Well most Econ courses don't actually require A-Level Economics so it doesn't matter that you won't have it. However, if applying to LSE/UCL etc then achieving a D in ASLevel Economics would lead them to question whether you are right for the course. Definitely resit AS-Levels, and they may also ask, why, if you're applying for Economics, you didn't choose to continue it to A2, surely when you should have a passion in it and want to learn as much as you can. Just think about that point as if you get to an interview somewhere that may be asked to you.