With regards to VFR on top......
Standard PPL privileges site VFR ie maintain clear of cloud by the margin appropriate to airspace, speed, type etc and viz requirements. So therefore VFR on top to me (and I interpret as legal requirement, but sensible also) means that you are able remain clear of cloud in the event of an engine failure. Whether legal requirement or not, pilot with no IMC training/experience since PPL course + engine failure + IMC is not a great scenario. I am no legal eagle so would not pretend my interpretation would stand in court.