but how does that change what appears on SVs as the time posted?
Because you haven't changed your time setting, the forum thinks that YOU are an hour ahead of real time. So the times the forum shows YOU are the times it thinks you should be seeing. Change your time setting to GMT and the forum continues to display what it thinks YOU should be seeing which just happens to be the actual UK time. Change it to GMT+4 and everything appears 4 hours ahead to YOU. The setting is unique for each registered user.