UK equivalent of high school diploma

[axa]

Does anyone know which UK qualifications (and how many) would be evaluated as equivalent to a high school diploma? 

For those unfamiliar with the UK system, we don't get any kind of school leaving certificate. Instead, we take subject-specific qualifications: GCSEs at age 16 and A Levels at age 18. 

I've Googled around and not sure if I'm looking in the wrong place but I can't seem to find a concrete answer; one person said it'd be a Level 3 qualification (i.e. A Level), another said three GCSE passes equate to a high school diploma, everyone seems to have a different idea.

Considering some US colleges offer credit for A Levels, and that school-leaving age was 16 when I did my qualifications, I'm guessing it's more likely to be GCSEs / Level 2 qualifications, but not sure how many.

Reason for asking: I'd like to pursue a degree through UMPI, and it looks like they need a high school diploma? For personal reasons it's looking like getting my hands on my GCSE and A Level certificates for evaluation will cause so much bother that it might be much quicker and easier to just get a new qualification for the sake of having a high school diploma equivalent.