Most of the FTTP networks are now deploying XGS-PON hardware, and even Openreach are testing. Only Community Fibre are selling services faster than 1Gbps.
The swap of OLT (head end) and ONT (home end) is relativelty easy once the optical fibre is in the ground or on poles, so its much easier upgrade than copper or coax networks.
and only in certain places, I'm only 3 miles from Central London and I cannot get the 3 Gbps
That's because the 3Gbps package got launched on the 4th of November 2020. If your area has had Community Fibre before the launch date or relatively short deployment after that date, you don't have XGS-PON that's why you can't order 3Gbps.
All areas from the last couple of years and now onwards that are deployed will get 3Gbps availability by default. We have 3 leaseholder's properties in Central London that went live a few months ago and all 3 of them have 3Gbps option when selecting package.
Of-course, I too have witnessed that in certain areas a couple of years ago when entering random postcodes in London some areas didn't allow selection of 3Gbps.
Those are areas that have had the privilege to get Community Fibre earlier than someone like myself due to faster wayleave agreements. The drawback is that at the time when it was deployed Community Fibre didn't put XGS-PON kit in the first place.
It is possible that in future you'll get XGS-PON upgrade in your area or even higher than that. I think what Community Fibre are doing is first having an ambition to cover all of London to the maximum coverage before upgrading the older builds with XGS-PON or higher.