I would imagine you may struggle to find either an employer or a student willing to spend their £12k on an IR with you if you are very low houred. Indeed you could well be teaching someone with more hours than you. When I was choosing a school for the IR I looked at what experience the IRI's had. The one I chose in the end had 1000's of hours commercial flying experience. And it showed in the way he taught.
From memory I think there is a way an ATPL holder with experience can become an IRI without an FI course. I will look it up when have time.