United Kingdom Split Phase Transformer / Centre Tapped Earthed (CTE) / 55V - 0V - 55V - How do you wire the connector?
My question is: Which pin is +55V, and which pin is -55V? I've been
looking everywhere for this information.

At a particular instant in time, one pin will be positive and the other pin will be negative and they will have the same magnitude. Ten milliseconds later and the opposite happens. It happens because the output voltage from a transformer is alternating and not DC: -

Short story: it matters not one jot.