Been with Three for over seven years now, subscribed to SIM-only unlimited data (except for the period when the plan was not offered). The service was excellent for 4G and 5G signals until about three months ago. Now it is mostly poor to rubbish, with small periods of great performance just to lull you into a false sense of security. The network checker for my area has been displaying a "fault" notice continuously but no information on the nature of the fault nor when it might be addressed. Needless to say contacting Three Customer Service is a non-starter - they are always "busy" whether T/P, chat or what-have-you. No wonder they want to merge with Vodafone...or because they anticipate merging with Vodafone?
