I'm not familiar with Uk requirements if they differ from Swedish but if you don't have an IR before you do the CPL you need to do 25 hrs. If you have the IR you need to do 15 hrs. I don't think you get any "discount" for the IR hours requirement.
Basically you could save 10 hrs by doing IR then CPL. I found the IR was a question of flying off the hours after about 25 or so, it's not really that difficult, if you think it is, perhaps accounting would be an alternative.
The CPL felt much the same, especially now that many of the maneuvers that were required previously don't exist, i.e. chandelles, lazy eights.
That's my experience although I'm sure there are a million other opinions.
/Hotprop