The ground exams for a CPL(A) are 12 and the ground exams for the ATPL(A) are 14. In practise people who only need a CPL still tend to do the additional 2 exams just in case they ever wanted to turn it into a full ATPL. I don't know which subjects the 2 are however.
Simply, an IMC will allow you to fly in IMC and also enter controlled airspace (some restrictions) under IFR. However, the benefit of an IMC on your PPL is that it lets you enjoy the privileges of your PPL without having to be as restricted by the changeable UK weather. But as most club aircraft don't have de-icing equipment, don't expect to go cruising the airways at FL80 at 95kts. ATC might get a little
Good luck, dm