I'm not a JAA expert (infact i know next to nothing about instrument logging under JAA :P), but it would seem quite odd to me that you'd log actual instrument time during VMC just because you had an IFR flightplan. In that case 99.9% of the crews of the scheduled airline flights would be logging actual instrument - and i don't believe thats the case.
In the US you log actual anytime weather or atmospheric conditions do not allow visual flight because, for example, the horizen is not visible. This could be due to flying inside clouds, flying over water on a moonless night, or just piss poor visibility. You don't necessarily have to be on an IFR flightplan to do this.
You log simulated instrument time anytime you have the foggles or another view limiting device on - regardless if your in VMC or IMC conditions. I did both my IR & CFI-I checkride in actual (IMC) conditions + a hood on and both times the examiners insisted i log as simulated as those were the conditions required.
I doubt this will be much different to JAA rules.