Good news...only 10 hrs
So, usual advice - its in CAP804. Relevant section is 1.4.L - Modular training course for IR(A).
Essentially you need your PPL(A) as a minimum. If you haven't done CPL(A) don't do it first because the credit works best if you do IR first. You need some x-country time on aircraft (10) and then at least 40 more on others. You'll also want your night rating on aeroplanes. Then its just 10 hrs training. Clearly you'd probably want a twin IR so factor that in too. No exams to do either although if future aim is ATPL(A) then you will need to do about 5...just not specifically for the IR.