Considering the IMC is a UK only rating i doubt you will get any hours off your IR, but any instrument training you get before doing your IR is a huge benefit, even learning how to fly straight level and perform rate one turns will help.
If you are looking for the cheaper option then I would suggest getting maybe 5 hours instrument time with a decent school and let them know that you intend to go on for the IR.
This way you get some IFT and save money by not flying all the IMC hours and sending off more money to the CAA for the rating to be added to your licence.
Most importantly, keep current and make sure you fly withing IR limits:+50/-0 feet on alititudes, +/-5deg on headings and tracking i believe.
Hope this helps
Nick