I wonder if the wrong engines have been put in UK golfs?
A golf is heavier than a polo... Heavier is more polluting. I'd guess the polo doesn't have the gpf... In the same way as previously the oil burning passat had adblue but the golf didn't (does now...)
The Golf isn't that much heavier than the Polo - the Polo GTI+ is about 56kg lighter than the new GTI Performance. My pre WLTP Polo GTI + doesn't have a GPF, the WLTP compliant one that started building in November does.
The Polo is marginally smaller than the Golf now ,- same interior size, shorter bonnet (Most of the difference) and slightly less deep boot.
There really isn't much in it at all.