I have an IR (albeit, SE IR) with a Class II medical + audiogram.
However, the FTO might have their own rule if they wish to ensure that all their candidates have the chance to progress to the commercial world. An FTO may well do this if they wish (for example) to advertise high success rates for their students gaining flying jobs. A successful student who subsequently failed their Class I medical would ruin their statistics, so hence their Class I pre-requirement.
Irrelevant what the CAA say if that's the case.
You would then have 2 options.
Take the Class I, and go to your chosen FTO; or
Choose another FTO that will allow you to do the training with a Class II