Most 3G is already gone. The few remaining networks still offering it (just O2 I *think*) are supposed to be switching off by the end of the year.Apparently there’s some networks switching to 4/5 G only so older devices will be useless.
3G devices revert to 2G for calls and texts (2G is staying around for a few years yet because the bandwidth/frequency requirements are much simpler and pagers and safety critical stuff still use it). So some 3G device users may not have realised that it is already gone from most networks. They won't be able to do internet once they're back on ye olde 2G. Calls and texts on 2G still work but it's 20+ year old tech and cannot do the call quality of the newer tech.
It's not just the UK - most of the world is turning off 3G and eventually 2G.
Time to think about new phones unless you really do barely use it.