There is every point to do the IMC rating (the correct name) and you may do it immediately after gaining a PPL;
25 hours total experience as pilot following the issue of a PPL (which may include the training for the IMC rating). 10 hours as Pilot in Command (PIC), including 5 hours as PIC of aeroplanes on cross-country flights.
A well conducted IMC course will provide a good basis for your IR at half the hourly price. When conducted properly to a good standard it will keep your expensive IR training to the minimum. Choose your school for the IMC rating carefully and where the IR course requirements are well understood.
You are required to gain a lot of PIC time before starting a CPL course. You could of course fly endlessly around in circles like a tethered chicken or extend your knowledge by exploring the UK and Europe and having some fun. Holding the IMC rating will give you a considerable confidence in marginal weather and ensure your safety. The safety record of 40 plus years of IMC rated pilots is evidence of this. There is nothing more pleasurable than cruising above 8 oktas of cloud in stable air, sunshine and clear blue skies, whilst below its cold, wet and miserable.