Re: Bristol has a 5G testbed
Distance is a good approximation over thousands of km's although the speed through fibre is approx two thirds of the speed in a vacuum.
Over shorter distances where actual cable paths maybe significantly longer and you have protocol translation (typically Ethernet-SDH-Ethernet) and routing/switching/repeating latencies, the accuracy is much less.
Finally, load on the circuit may also contribute to delays that area higher percentage of overall latency on short circuits.
So London to Guildford is likely to be 0.5ms +/- 0.5ms as an estimate before it goes in and you can test.