If you take PompeyPaul's post and answer the questions he asks, as he asks them...
"So when do you switch to FL ? Or don't you ever ? Why did the books seem to suggest about 3000ft you switched to FL ?"
So when do you switch to FL?
Above the Transition Altitude (Whatever that is for the area you are in)
Or don't you ever ?
Depends where I am... I fly out of Popham, and London TMA and Solent TMA are 6000ft, so hardly ever fly FL as I don't fly higher than about 4000ft, and by using altitude I can ensure I remain OCAS. I do though if I am above 3000ft and crossing the channel, just be sure to call VFR on you initial to the French otherwise they always seem to assume IFR with a FL.
Why did the books seem to suggest about 3000ft you switched to FL ?
Because for most of the UK the TA is 3000ft. and normally outside CAS it is 3000ft unless a different TA is stated.... AND the less well known, (paraphrased) "Under Controlled airspace where the base is stated as an altitude" (E.g. under some of the Airways into and out of the London TMA whereby they are quoted at 5500ft. Would mean the TA is 6000ft - Otherwise you would be FL above 3000 and there would be no real way of ensuring you did not become a statisitic)