Do all a level subjects matter?

I think I made a mistake, i’m in Year 12 doing Maths, Further Maths, Econ and Design. I don’t have a clue why i chose design because I want to do econ/maths in uni and am looking at LSE. Have i ruined my chances by taking design instead of an academic subject for my fourth alevel?
GCSE: 9999888887 (A in FSMQ)
alevel predicted: 4 A*s

You haven't "ruined" your chances, no. They've taken a little knock, perhaps, but that's it. If you'd been doing three A levels and one had been Design, things might have been different, because it's on their list of common "non-preferred" subjects.

However, they say on the same page that "We consider traditional academic subjects to be the best preparation for studying at LSE. We expect applicants to offer at least two full A-levels or IBDP Higher Levels in these subjects (although typically, applicants will apply with three or four)".

Their list of "traditional academic subjects" is on the same page and includes, as you might expect, Maths, Further Maths, and Economics. So you meet the "at least two full A-levels ... in these subjects" criteria, and actually take three.

Some other applicants will be taking four. But they may not have as impressive a GCSE profile as you, or may not have A*A*A*A* predicted, or may not have as impressive a personal statement or reference. So you're still in which a chance.

(You presumably know that for BSc Economics at LSE they have 15 applications per place, and for both BSc Mathematics and Economics and BSc Mathematics with Economics they have 9 applicants per place.)

I think I made a mistake, i’m in Year 12 doing Maths, Further Maths, Econ and Design. I don’t have a clue why i chose design because I want to do econ/maths in uni and am looking at LSE. Have i ruined my chances by taking design instead of an academic subject for my fourth alevel?
GCSE: 9999888887 (A in FSMQ)
alevel predicted: 4 A*s