Flying Dutch
Sorry - but wrong. Flight Levels and quadrantal rule are only mandatory for IFR flight above the transition altitude. If you're VFR you can fly at whatever level you like because at the end of the day you are just that - VFR and so have the right (and sometimes the need) to change your level to stay VFR (Rule 30 is the relevant rule for IFR, there is nothing similar in the VFR rules - 24 to 27)
BUT
If you're flying close to the base of CAS you should be on the same pressure setting that the base is measured from - 1013 if it is FL's, the AERODROME QNH (not the regional, this will put you too high) that the base is again measured from (London QNH in the case of the the London TMA, respective aerodrome QNH if passing under an airfield CTA.