Depends where the transition level is on the day (the transition layer changes with pressure alt), if you were flying IFR in the UK you would change to 1013.2mb/hpa at 3000' QNH.
Now, if you are VFR, you can do that (and fly quad levels etc), but I have flown on the QNH around and about at 4000' quite happily.
That's my understanding, but I'm probably wrong (I don't have the book to hand) so someone, please put me right!
