There is no requirement for me to be able to see the surface or any obstacles (which could be in cloud). I am still VFR and am using the QNH to ensure MSD from the unseen obstacles, terrain and built up areas.
DFC
But if you comply with the ANO, then you have to be 1000' vertically from cloud, and so will never come close to any unseen obstacle in the cloud. The pressure setting is irrelevant since however you determine your vertical separation from the cloud, it still requires 1000' minimum separation distance for you to be VFR.
If you are using a reported ceiling as your datum to calculate your 'separation', then you coud still be breaking the law as it will not be a uniform figure and could change at any moment from the figure you are using. But obtaining hard evidence and prosecuting you could be tricky
