The privileges of all licences and ratings are in Schedule 8 to the ANO.
The privileges of the IMC rating extend the privileges of a UK PPL and BCPL holder to fly out of sight of the surface and under IFR in Classes D, E and F airspace.
The holder of a UK CPL and ATPL does not have this limitation to the licence privileges, and so do not need an IMC rating.
A JAR-FCL licence is slightly different, in that unless you have a valid instrument rating for the aircraft being flown you have no IFR privileges and thus may be in need of an IMC rating to fly IFR on the PPL privileges.