The problem with unit grades is that they're not standardized across universities, so what might be a first at a lower-tier university might be a 2:2 equivalent at a top-tier university.

Hence companies relying on A-Level scores and university reputation when filtering.

What ECs do you have ? - that's probably your best bet of getting visibility. Assuming you're a second year student you might want to apply for a GSoC on a low-level project.