I have 3 as my phone carrier with a high end modern phone (pixel 9 pro), so my experience shouldn’t be limited by hardware.

I’d say in a city (Manchester, Liverpool & London mostly) I get 5g about 30% of the time, that’ll get speeds around 50-80mbps. The rest of the time I get their oversubscribed 4g which struggles to do 1mbps

The more rural you get the chances of 5G shrink to like 5% of the time, though the 4g speeds improve a bit closer to the 10mbps mark.

Of course YMMV, but it might be worth getting a 3 PAYG ESIM or something on your phone just to see what the signal is like by you