Norunway has it correct. Having just had to do the 7 IR theory exams, it's a set course. Contact GTS, Bournemouth, as they do the course. I believe there is only a couple of course providers for IR theory in the UK.
As for the ATPL theoretical knowledge instruction discretion, I believe this refers to exactly that, and not 'which' exams. The full IR course exams have to be sat but the amount of study required is at the discretion of the FTO. Basically means that you don't have to complete the 500hours or whatever ground school attendance before sitting the exams.
