To sum up.
VFR on top is only allowed provided that:
a. Your CofA/PTF allows this.
b. The airspace allows this
c. Your license allows this
As far as (a) is concerned, there do not seem to be any general or specific rules for Permit aircraft. But you would have to check the provisions of your specific PTF to be sure.
As far as (b) is concerned, see above. For each class of airspace there are specific definitions as to what constitutes "VFR", and sometimes this does include an "in sight surface" restriction, sometimes it does not. May also depend on the altitude.
As far as (c) is concerned, there is a specific provision in the UK ANO that a plain PPL, who does not have a (current) IMC or IR rating, has to stay in sight of the surface at all times. This applies to all PPLs that are issued under the ANO (in other words, UK issued PPLs, JAR or National) and is a license restriction, so it applies worldwide. Obtaining an IMC or IR does lift this requirement, worldwide. (Even though the other privileges of an IMC - IFR flight - do not apply worldwide.)