The coverage checker on the Three website is normally a good indicator, just put in the desired postcode and it will tell you the coverage for 3,4 and 5G.
In terms of bandwidth it will depend on a few factors like time of day, location and device. However on my S21 I have experienced 5G speeds from ~190 mbps to ~1040 mbps, there are a few speed test apps and websites you can use.
Sites Mentioned:
Three UK Coverage Checker - https://www.three.co.uk/Discover/Network/Coverage
