In a nutshell:
As from 1st July, you will only be able to add an IMC rating to a CAA licence - you won't be able to add it onto an EASA licence (which a JAA PPL will become), and you won't be able to use it on EASA aircraft.
So - if you want to fly anything but permit aircraft and exercise the privileges of the IMC rating, you need to get it on your JAA/EASA licence before the 30th of June.
If you want to fly a permit aircraft on a CAA UK licence with an IMC, you can add that to your licence post 30th June.
The IMC itself will be valid until 2014 - after that, it will cease to exist. As far as EASA licences are concerned, there is talk of grandfathering/converting the rating to some form of UK only restricted IR.