Does that mean you can only fly VFR on top with an IMC
An IMCR or IR pilot can fly VMC on top, above a solid layer, and call himself either IFR or VFR, as he chooses.
IFR is usually preferred because he is much more likely to get a RIS that way, not to mention things like overhead transits (ATCOs often seem suspicious of the nav skills of a "VFR" pilot)
A PPL-only can fly above clouds but he has to be within sight of the surface. I am not aware of a specification for the degree of overcast (X octets) above which this is not allowed - is there one?