Gemma, you are quite right.
One could be approaching some airfield which is reporting "CAVOK", but which is OVC060 and if flying VMC on top, you can't legally descend while maintaining VFR.
This is rarely a problem in the UK because there is so much controlled airspace at/above 5000ft.
It is a real issue when going abroad, say France, but then a UK/JAA issued PPL holder cannot fly out of sight of the surface anywhere unless he has an IMCR or an IR.
And if he has the IR he might as well be on an IFR flight plan and be done with all this "VFR nonsense". Unless he is > 2000kg.....
The traditional VFR solution is to find a "hole in the cloud"
and descend through that. Just don't do it in the overhead where everybody will see you popping out of the cloud. Obviously your navigation has to be good!
Another CAVOK gotcha is that you can have unlimited wind. Another one is that horizontal vis can fall down to 5000m before the CAVOK status is revoked; this is OK for radio nav but no good for visual nav unless you know the area well.
Minimum Safety Altitude (MSA) is (in the UK) defined as the highest obstacle within 5nm of track, plus 1000ft, IIRC. The Sector Safety Altitude (SSA) is something else; it is the number printed in the middle of each map square, and represents the highest point within that square (plus a little margin extending into the 8 squares adjoining), and adding 1000ft to that figure should do the job. The SSA+1000ft is safer than the MSA.
The whole issue of terrain clearance and VFR is clouded because many people think "I am flying VFR, therefore I am visual, therefore I don't need to plan the altitude". This is OK until you enter IMC, either deliberately or accidentally. IMHO it's better to plan every flight as IFR and fly it as planned. However, one can't do sightseeing or bimbling that way.