@Edent @afink Much of London can now get FTTP from either Openreach or Community Fibre but there are gaps. The latter supports #IPv6 and on Openreach you could go with BT/EE, Sky, AAISP, Zen or even Vodafone who are rolling out IPv6 currently.
If you are in a flat/apartment then Hyperoptic would likely be a good option too.
Other very local providers exist too.
These are two useful resources:
https://labs.thinkbroadband.com/local/broadband-map#12/51.4893/-0.1478/geafttp/community/
https://www.thinkbroadband.com/packages
There's a 'Supports IPv6' filter under Advanced
@afink
Have a read of this
@Edent wow. DOCSIS is waving from the middle ages... what a bummer. send them faxes to complain! they probably still read them