my UK Polo (3 door) had proper 165mm speaker mounting points in the rear

What year model is your Polo (possibly even month)? Mine was made right after the factory changed the tooling from '00 to '01 models (clear glass headlights, 3rd brakelight in the trunk lid frame etc)

Might be that there are suitable holes under the rear side panel - haven't looked... How high are they placed? And how far back from the edge of the door opening?

/Michael