Andrew Erickson wrote:

Are the secondaries of UK power distribution transformers center-tapped 440V
windings, with
different houses getting opposite phases, or just a single 220V winding?

230V actually and quite often still 240V which is what is was in the first place
before Brussels decided it had to change for harmonisation reasons..

The 230V supply is actually a single phase of a 415V ? 3 phase supply. I believe
that the houses in a street are connected to the phases sequentially so the first
property will be on 'red' phase, the next on 'blue', then 'yellow' them back to red
again and so on.

Graham