You'll find the info in CAP53 - guide to PPLs etc and also in the ANO. The IMC rating is strictly UK so it doesn't appear in any JAA pubs.
You do have one by virtue of your UK ATPL.
It give you the right to fly in IMC, under IFR in any UK airspace except Class A (to be honest I don't know whether you're allowed above FL245 but I guess that's irrelevant). There's something about adding an extra allowance to your MDA/DA but I don't know off-hand how much.