I've been wondering the same thing for some time - I asked the question at the Flyer show a year or so ago and was made to feel like some kind of idiot by more than one FTO!
I've just trawled through the latest copy of LASORS (being careful to distinguish between the 'Commercial Pilot's Licence' and the ' Basic Commercial Pilot's Licence') and I found this section from page 52......
Commercial Pilot's Licence (Aeroplanes)
Minimum age - 18 years
Maximum period of validity - 10 years
Privileges:
1 The holder of the licence shall be entitled to
exercise the privileges of a United Kingdom
Private Pilot's Licence (Aeroplanes) which
includes an instrument meteorological conditions
rating (aeroplanes) and a night rating (aeroplanes)
or night qualification (aeroplane), and.........
This seems to suggests that you can fly UK PPL IMC under your CPL. Not sure why it states UK PPL though rather than JAR, as IMC rating is only valid in UK anyway?
I guess that you really need to speak to someone at the CAA to get a definitive answer. If you do, please post here as I would love to know too!