Came across this article while doing my quarterly google news hunt for any info..
www.standard.co.uk
So sounds like a fair bit of Northern and Central will be getting coverage immintently. I also wonder if this will include the Elizabeth line, given that tweet from TfL saying it was coming 'soon' after opening.
Regardless, the article does confirm the latest EL could get phone coverage would be end of 2024 (which is the first time I've seen it actually written!). I would assume a lot sooner, but great to know.

Tube passengers may be able to stream World Cup Final on Underground 4G
The exact launch date could be ready in time for the final on December 18

Tube passengers may be able to stream the World Cup Final on their smartphones while deep underground, it can be revealed.
The roll-out of a 4G wireless network on the London Underground is due to hit a major landmark by the end of next month, when a number of stations and tunnels on the Northern and Central lines will be the first in central London to be connected.
The exact launch date is being kept under wraps but it is understood it could be ready in time for the final on December 18. Stations previously earmarked as being the first to benefit include Euston, Bank and Oxford Circus.
The Standard has been given behind-the-scenes access to the work being done to install super-fast wifi to the subterranean sections of the Underground. The 4G network will exist alongside the existing wifi on the Tube.
So sounds like a fair bit of Northern and Central will be getting coverage immintently. I also wonder if this will include the Elizabeth line, given that tweet from TfL saying it was coming 'soon' after opening.
Regardless, the article does confirm the latest EL could get phone coverage would be end of 2024 (which is the first time I've seen it actually written!). I would assume a lot sooner, but great to know.