The UK has been subject to EASA Regulations which will be incorporated into UK law from 01 Jan 2021. The requirements for Instructors can be found in the Easy Access Rules for Flight Crew Licensing (Part-FCL) Subpart J which includes the requirements for Instructors commencing FCL.900.
If you look at CAP1721 it will list an alternative means of compliance which reduces the FI Course to 30 hours of ground training and 15 hours of flight training followed by an Assessment of Competence. To add the IRI and CRI(ME) you will need to complete the courses listed in Subpart J.
A point to consider is what do you want to teach? PPL or Commercial, the latter may present more of a problem as the UK will not be an EASA State after 31st December and to train for an EASA licence will require an EASA instructor; UK instructors will no longer be EASA instructors.