O2 and Voda...
So the two old names (well O2 was Cellnet) show all the signs of being able to handle change and maneuver to cope pretty much on-par with an oil tanker.
All the flexibility of a gymnast with rigor mortis.... etc etc...
There are certainly villages in my "East of England" parts which are lucky to get 2G from O2 and Voda.
BTW, Ofcom have an app which should track all this bad coverage, and allow them to provide a very accurate picture.
https://www.ofcom.org.uk/phones-telecoms-and-internet/coverage/ofcom-mobile-research-app