You’ll need to sit and pass all 13x CPL or ATPL exams but don’t actually need to obtain a CPL to teach ab-initio PPL. Without the exam passes you’ll be limited to teach for the LAPL only with regards ab-initio training.
A Class 2 medical will be fine even if you wish to obtain an IRI and/or CRI(ME) in the future to teach the ME-IR course (assuming you have an ME-IR as well on your PPL). The only time you would need a Class 1 is if you were going to teach the CPL course and then you’d require a CPL or ATPL in any case.
You will need to attend a Flight Instructor Course and pass the Assessment of Competence, average time to complete the course is ~6 weeks and involves 125 hours of ground school and 30 hours of flight training. This course is exactly the same whether you are to teach only the LAPL or the LAPL/PPL (with CPL exam passes).
Once you have your FI certificate you’ll initially be restricted so no first solos or first navigation solos and you’ll be supervised by a fully unrestricted FI. To remove the restriction you’ll need 25 solo sign-offs and at least a hundred hours of flight instruction given.
On-Track Aviation have a good breakdown on FI requirements:https://www.ontrackaviation.com/fi-aeroplane.html
Standards Document 10(A) is a great introduction to the FI course, details AoC expectations, and within it are 250 questions that really have you thinking about them, these form the oral portion of the AoC. In preparation for my FIC, I have just finished going through answering them by researching and writing my answers down in a document, 137 of A4 pages in total! My answers are probably a load of waffle though but has helped cement some of the theory into my head.https://publicapps.caa.co.uk/modalapplication.aspx?catid=1&pagetype=65&appid=11&mode=detail&id=1202
On-Track Aviation also sell their instructor manual separately which is a fabulous document.
The CRI (SE) course is also an option…. Fabulous course which takes around a week and once you pass the AoC will allow you to teach anyone who holds or has ever held a licence. Differences training, hour with an instructor, SEP revalidations etc. No ab-initio training. You don’t need the CPL exams for this either.https://www.ontrackaviation.com/cri-land-se.html
To add privileges to teach the Night Rating will require a couple of hours ground school and an hours flight with an FIC Instructor, this can be completed whilst restricted and can even be part of your initial FIC if you time the dark nights right.
Whatever you decide all the best and keep us informed!