A few weeks ago I raised this point, standby before or after a flight duty, with the CAA flight ops inspector for my company, (which i suspect is the same one as yours) while being ramp checked as i had heard some colleagues say that it was illegal.
The CAA chap said it seemed pointless to put you on standby before an FDP as it needlessly wasted duty hours, but that it was not illegal if the company wished to do so.
In my experience, one is very rarely called in from standby before or after an FDP. I believe the practice of putting you standby after an FDP has come about after some crews declined an extra sector when asked, so now they can force you to.
Also, one cannot be seen to have too much time off as this upsets the boss, even when there is no chance of a flight.