Agree with what Bose said except I think he missed your statement of your total hours. You already meet the requirements for the FI course except that you must have 200 hours of which 150 must be PIC. You may also need to do the 300Nm triangular cross country - check your log book as you may already have done this in the past. All the requirements are in CAP 804 under FCL.915
Given your long lay off you might want to consider some consolidation and personal development flying to take you up to the required total. Take an aerobatics course; get really familiar with stalling, slow flying and spinning. Try a tailwheel conversion. Do some flying into short farm strips.
Once you have the hours and are fully up to speed and confident in your own aircraft handling then go straight to the instructor course and good luck with it.
Happy landings
3 Point