Interesting discussion not helped by the UK and JAA differences which does cause some problems. That said I shall put my commercial instructor hat on and try to help you through the minefield.
You elect to fly under either VFR or IFR rules.
Whether you CAN fly under these rules is determined by the VMC minima (as stated in previous posts) AND what type of airspace you are going to be flying in and your qualifications.
If the actual met. conditions are below the required VMC minima then IMC exists and you MUST fly in accordance with IFR. The whole point being to make sure that you are above MSA at a suitable FL so you don't fly into cumulo-granitas. So as a basic PPL make sure you remain visual otherwise you might not live very long.
That the other end, flying on airways (class A) on a gin clear day, though your conditions are VMC you MUST fly in accordance with IFR. Class A - NO VFR permitted EVER, the SVFR given in some control zones is an ATC concession and must not be assumed as a right. I have known ATC refuse SVFR.
In theory, a basic PPL could file an IFR flight plan (given VMC) in airspace where full IFR requirements aren't needed however there isn't much practical point.
There is no such thing as VFR at night. Your flight conditions might be VMC but the flight must be carried out under IFR, en-route obstacles again. (SVFR to clear a zone).
You USED to be able to fly "VFR on top" ie above layer of cloud, but as stated in ANO 8 a basic PPL must remain in sight of the surface at all times.
Summing up :-
VMC conditions fly either VFR or if required and qualified IFR.
IMC conditions MUST fly IFR - therefore be qualified.
Once you have experienced flying IFR and got a full IR which allows you in the airways system you shall wonder why you ever bothered with VFR. Trouble is it's expensive to get there!
Regarding landing QFE or QNH. The UK are almost alone here and certainly for most commercial operations QFE is NOT used.
All IFR charts have obstacles and procedures assuming that QNH is going to be the prime reference. There is a rule that does require that on departure and upon a missed approach at least one altimeter MUST be set the airfield QNH. Changing from QFE to QNH whilst carrying out a missed approach is something you can well do without.
Jau - though your book says radar controller reads out height, you just inform him (probably knows anyway) that you are landing QNH then he will read out altitude for you.
I learnt using the QFE method but know I am a firm convert to QNH approaches, but you must bear in mind airfield/runway elevation. Not a problem throughtout most of the UK but interesting at Saltzburg at 1600 ft AMSL with the mountains - best go at night if easily scared!.
jau & others hope this was of some use.