Vodafone has had contiguous 4G coverage from London to Gatwick for some time, but south of there it can be patchy for all the networks.
The train has a better chance of getting a signal thanks to roof mounted antennas, but also needs to be equipped with the right modem to aggregate and access all bands. Band 20 (800Mhz) might be best for coverage, but also has limited spectrum that won't share very well (5MHz on EE and Three and 10MHz on Vodafone and O2).
5G upgrades will be good when more networks enable 700MHz for 5G, or 900MHz, which has the benefit of covering wider areas and still providing a good service.
Combining multiple bands will help further, but that will mean equipment upgrades for existing trains as I doubt any have 5G access right now.
EE and Vodafone are rolling out 5G quite rapidly and to wide areas.
3G is horrible and wherever possible I force my phone to use 4G and NR/5G only because a poorer signal for 4G will likely still work better than 3G, a flawed tech that is best forgotten as soon as possible. It's bad enough when you're still, let alone moving.