Interesting post, strepsils. However, as I understand it, under UK law all duty before a flight is Flight Duty time- even standby duty if one is called out. Depending on the length of the standby period, it may be permissible to add the length of the stand by to the permissible max flight duty time, but the whole duty counts as Flight Duty.
If one actually reports for work (even if for office duty), then flies later in the day, the entire duty counts as Flight Duty time, with all applicable limitations counting from the original report time. However, if an individual opts to report before rostered report time, that does not change the actual report time. It is up to the individual to decide how early to report for work. I tend to worry about how much rest an individual has had, if they are in the crewroom more than 15 minutes before report time for an early.