In the UK carriers are using n78 (3500MHz) for 5G. O2 is looking at rolling out n40/band40 (2300MHz) at some point, all of which are support by all iPhone 12 regional variants.
There is an additional auction for long range 5G on 700MHz in the UK and parts of Europe (Italy?) but this isn't being used yet. According to
https://www.apple.com/uk/iphone/cellular/ this band is also supported on on regional variants (n12 and n28).
Europe and Canadian model iPhones don't support mmWave (above 6-GHz) 5G. At this stage, there is no mmWave in Europe, although the EU (and UK?) are looking to license 26Ghz (n258) mmWave at some point. However, the US model does not support n258 - it supports n260 (39 GHz) and n261 (28 GHz) which are not in the pipeline for Europe. So the additional bands of the US model are pointless outside the US.
I note there also is the 600MHz 5G band which is supported in the Canadian (and US) version, but not in the European one... Not sure what the pipeline is for this?