... but the IMC gives you no credit at all for the IR.
Unfortunately, the IR is a 55h course (multi engine), and there is now absoultely, positively no reductions at all. Not even if you have 20,000h of IR instructing time under a different authority- it's still 55h (and £13K's worth) of training for you!
Q. 2... The IR is the same for a PPL as for a CPL. The advantage of doing it before you take your CPL course is that the CPL course minimum requirements come down from 25h to 15h. But, bear in mind you need to have had the rating issued. And in order to have had the rating issued, you need 200h TT.
150h can cost you a lot of money, but the money need not be wasted. Most of my hours are done in the USA (at about half the cost of hours over here). I also did a ME rating over there, and some ME time afterwards. I know that this will have helped me, because the instructor I flew with really wanted me to progress during my hour-building. Without this, I *know* that I would find the IR course a whole lot harder. How much I improved my ME flying is not for me to say!!!
So the short answer to your Qs are that yes, it is a lot of money when you might not get exemptions/ reductions. But... put it into perspective.