The current core rules for VFR below FL100 outside class A airspace in the UK are
EITHER (A) 5,000m forward visibility, 1,000ft vertically and 1,500ft horizontally clear of cloud,
OR (B) if a helicopter at or below 3000ft amsl, 1,500m forward visibility, clear of cloud and with the surface in sight (my bold).
'With the surface in sight' is defined in UK legislation as
...the flight crew being able to see sufficient surface features or surface illumination to enable the flight crew to maintain the aircraft in the desired attitude without reference to any flight instrument
So If you have a low level undercast/fog up to say 1000ft with gin clear skies above, and you fly at 2000ft or above then you're VFR, and even if you fly down to the top of the undercast, then provided you can meet the definition of 'the surface in sight' above, you are still VFR. The pictures earlier in the thread eg. of the O2 poking through the fog IMHO would meet the definition, depending on the direction of flight.
You can use the words 'on top' if you want to qualify your situation although there's no technical reason to do so since those words are not used in UK legislation.
Of course whether it's sensible or legal to do any of the above depends on the ATC service you are receiving, the airspace around you, the equipment in the aircraft, the nature of your flight, your own qualifications, and the plan you have for getting back down.
As far as I know the EASA Single European Rules of the Air are not intending to change these core aspects of the rules.