ScouseFlyer,
The UK CAA restricts holders of a UK PPL or a UK-issued JAR-compliant PPL a bit further than the JAA regulations proscribe. According to the JAA regulations for a PPL, you can fly VFR-on-top (airspace permitting) but the UK CAA won't have that.
The IMC rating, in addition to giving you IMC privileges within UK airspace, also lift the restrictions that the UK CAA places upon your bare PPL.