As maost UK controllers are usually available in the evening, when it's dark, is there any issues with changing the time of day rather than always flying at night?

None at all.SOme people advocate putting something like "daytime set" or "daytime flight" in remarks but its completely optional.If it were to be enforced imagine those up near the polar cap - they would have dark flights for 6 months of the year!

Or people like me who are just in the dark all the time would be lost :)

There is a complete disconnect between SB and your FS computer. You can fly at 10:00PM local time in New York, showing a 747-200 in American Airlines livery and on your FS machine actually fly a 727, in the day with a Eastern Airlines livery and as the Vatsim world will only see you as the American 747. I am not advocating this be done but it illustrates to point.Mark.