That post was mainly to give an background overview, but now to answer your questions directly...
Is it likely to be 2023 or later before we see 4g+ in most UK towns?
Who knows! The Huawei ban has probably put delays on things, however just saying 'town' is a pretty wide sweeping; if one site in a town has an LTE-A feature deployed would that define it as being deployed to a town?
Is 4g+ a prerequisite for 5g in most cases?
No (but in practice, it probably means 4G+ will be available if 5G is added)
Fepending on the provider's current deployed hardware at a given mast that is planned to get 5G radio and antenna changes
may be required. For Three, for example, their old 3G/4G radio deployments are Samsung units and at the moment 5G requires 4G hardware from the same radio provider. Three's plan was to use Huawei for their 5G deployment so that requires 4G Huawei hardware too. In the most basic upgrade deployments they are replacing the Samsung radios with Huawei units and they have also chosen to also re-farm some of their 3G spectrum on B1 (2100mhz) over to 4G too, which enables CA.
Additionally, 5G in the UK is currently non-standalone (NSA), which does need a 4G anchor to be able to 'add' a 5G stream to the connection. In the future there'll be 5G standalone (SA) which will not need any 4G present.
As I understand it, Three is upgrading some towns with 4g+ capable Radio masts, but there isn't much information about it right now. Link here:
http://www.three.co.uk/hub/network-...809_81706312a434f5913edbd11f157ccf2e&aidset=1
You likely won't see much info about a providers ongoing plans (which are likely delayed/derailed with the Huawei ban and COVID).
Peter has detailed the variations on site upgrade 'levels' that Three were doing/planning here:
Three UK's Project Chariot Huawei Rebuild, including 5G
pedroc.co.uk
I probably won't upgrade my phone for a while, especially since it already has support for LTE cat.6, and therefore supports '4g+'.
That's totally your choice. If it does what you need then there is no need! It would also be a 'waste' to upgrade if none of the LTE-A features were deployed to the areas you visit (and use data at).
I haven't yet found any info about if or when EE will upgrade more towns to 4g+. I assume they are focused mostly on rolling out 5G to cities.
Same as above, you're unlikely to hear about technical details of planned deployments. Sometimes if the provider has to get planning permission your council planning website may show the plan documents for what antennas/masts are due to be replaced and this can give an indication of what a new site would feature.
Can anyone let me know what download speeds they get with 4g+ ? How does it compare to a Fibre (FTTC) connection in terms of speed and latency?
Theoretical speeds vary massively, depending on provider/bands/spectrum/lte-a features. Practical speeds even more so, as that depends what is available from the mast you're connected to and other factors (noise/interference/current load/backhaul).
Comparing from person to person, place to place isn't really viable. If you need to know what speeds are in a given place you need to figure out what the connected mast/provider combination supports and test things out.
Also, does anyone have any info regarding when Three is planning to complete their upgrades to 4g+ in towns?
As mentioned above, you're unlikely to find this. There was a plan and I remember seeing a few press releases giving broad timelines for some things but Huawei/COVID will have changed those.