I have a work phone on O2 and it's shockingly poor in my area. Strong 4G+ signal but so slow it's unusable. I assume it's over subscribed or an issue with the nearby masts.
I have Three ony personal phone. This is good where I live but move towards the town centre it grinds to slow speed despite being a strong signal.
It really does feel like there's just too many devices and not enough capacity for them.
Separate note. I read somewhere (and I forget where) that a high number of 5G masts are being rejected planning permission by pressure put on by local residents. It's something mad like 90% are denied. There was one planned right next to where I lived but people started putting up posters about it looking ugly. The public notices disappearee so I assume the residents won. The location was right next to a roundabout and close to a railway. I really doubt it would make a different to their view.
There's still a bit of graffiti around which is on the same page as that so I can believe it!