How to make the best possible internship resume??

I assume a 1st class 1st year GPA (or at least 2:1) (or Freshman year 3.8+ in the US) would be essentially a prerequisite for any chance at an elite internship , except that, what actually awes the CV reviewers at the HR dept. at MBB/T2 in a penultimate year kid's resume.

I am currently a michaelmas term international student at a top target for the country I am studying in for IB which is also a decent target for Consulting. I am currently a Philosophy student but I am thinking of changing to PPE because both are in the same dept so wont be the most difficult task to migrate and I have a great interest in Politics and a good amount of interest in Econ as well.

Will joining relevant clubs, trying to gun for leadership roles and looking for michaelmas term pre-internship programs be good enough? Also I am pretty much a generalist, like I am doing a qualitatively heavy course but I do have a strong CS background {Student Head of CS dept. for my school of more than 4000 kids and Excel-PPT/Coding/Audio-Visual Editing experience), will that be looked favourably or will a particularly spiked candidate be looked more favourably, cetirus paribus.

Also, do they look at A levels and GCSEs (in my case my native country's board examinations) and Test Scores like SAT, GRE, GMAT, STEP etc.? And since I will also be applying for IB interships in the meanwhile, will exams like SIE be helpful?