OK, you want the Roche Elecsys Anti-SARS-CoV-2 S test, where S stands for Spike Protein. Testingforall is one such UK provider, but anyone using that equipment. Very important to have a quantative rather than qualative test, and one specifically for the Spike. As you rightly indicate, a negative reading doesn't prove very much, so I wouldn't take it too seriously.

Oh and allow 15 minutes at least to take the test, it's a bit messy, you somehow need to get up to 20 drops of blood into a tube and it's not as easy as it sounds.