Roughly 15 metres. Almost certainly a cabling issue, although frustratingly the electrician who installed it says that they think it's fine. I've got a ubiquiti U7 AP connected to a USW lite 8 via POE, and it negotiates GbE, but then falls back to E.

I solved this yesterday. It was in fact a cable problem. I think the initial GbE negotiation was a red herring from ubiquiti as two of the pairs were flipped.