The IMC rating is a relatively straight forward rating that has a minimum of 15hrs flight training and a ground exam, which you would take in similar cirumstances to any of your PPL exams (if you have taken any).
The IMC rating allows you to fly IFR but there are limitations in what airspace you can fly into under IFR. People will probably come out with that in a post no doubt.
The Instrument Rating at present consists of 7 ground exams that must be taken at the CAA office in Gatwick and a minimum of 55hrs flight training in prep. for the IR. Basically it costs a fortune and takes F**ing ages. I'm almost certain you don't have to have an IMC rating prior to trying to get an IR
BUT there is a loophole...you can get an FAA (North American) Instrument Rating which must be done in the US but it's pretty straight forward just one multi choice ground exam and a flight check ride and your away. You can only fly IFR in the UK/Europe if you are in a FAA reg.'d machine.
hope this helps mate.