There is an error in the ANO which is waiting amendment. The IMC privileges are exactly the same, irrespecive of whether you have a JAA or a CAA PPL.
The ANO appears to give the JAA licensee IFR privileges in D and E airspace only, F and G have been omitted.
As for an instrument rating: Instrument Rating (aeroplane) shall entitle the holder of the licence to act as PIC or co-pilot of an aeroplane flying in controlled airspace in circumstances which require compliance with the IFR. That means all Classes of Airspace.
The big difference between the CAA and JAA licences is that with a JAA licence you may only fly VFR whereas with a CAA licence you may fly IFR without any instrument qualification provided you fly within the limitations of the licence.