Instructing is fabulous and it is absolutely brilliant watching your students develop, flourish and gain what they set out to do, there is nothing quite like it.
I love instructing but I instruct part-time around a non-aviation well paid full time career and I most definitely could not give it up to instruct full time as much as I would love to do so! At the (true members) club I am a member of and instruct at we are all volunteers with no full time staff. All instructors are either retired military/airline pilots or current airline pilots with only me in a non-aviation career outside of instructing. I have two main students, we don't really have any more than two or three each as its not fair on either us or the students, we can dedicate more time to them.
The going rate for instructing is £30 per hour at our club and at the face of it that sounds like a very good hourly rate. However, instructors in the UK, at PPL level are generally paid only for hours flown so if you are at the club all day and fly three hours you'll be paid only for those three hours and out of this £30 you'll probably be responsible for your own Tax/NI etc. 3 Hours a Day, 5 Days a Week and accounting for 4 weeks holidays would give you £21.6k per annum or ~£1.5k per month after Tax/NI. But that is assuming the weather will play ball 5 days a week for the 48 weeks in the year you are at work. It won't. Especially in the UK. You'd be lucky to probably fly 500-600 hours in a year which would be ~£18k per annum before Tax/NI is deducted. You'll be knackered doing that too, its hard work but very enjoyable! You won't be applying for a mortgage on an instructors salary teaching at PPL level.
Where instructing starts to be better paid and/or salaried is when you become more experienced and have additional ratings so that you can teach the Multi Engine Instrument Rating. This is possible with a PPL/IR and an FI certificate with Multi and Instrument privileges. However, most ATOs (commercial schools) will expect you to be able to teach the CPL course as well as the ME-IR course which needs a UK CPL licence and a valid and current CAA UK Class 1 Medical. There may be niche schools out there teaching just the IR and you may get lucky finding an ATO to take you teaching everything bar the CPL course but they would be very few and far between.
Teaching type ratings full-motion simulators will require you to hold the type rating, a TRI certificate plus a lot of experience in a multi crew environment, for which you'll need to have been flying in the airlines (or similar) for a fair amount of time.
To obtain an FI certificate in the UK (we are not known as a CFI as that is an FAA term and a CFI in the UK is the abbreviation for the Chief Flying Instructor aka your boss(!)) you require at least a PPL and 200 hours including 150 hours Pilot in Command time if applying with a PPL (holding a CPL this reduces to 200 hours including 100 hours PIC). Once you have the experience you can attend an FI Course (FIC) which takes around 6 weeks with an Assessment of Competence at the end of the course. I found the FIC tough but the best course I have ever done, they expect you to be flying to CPL standards already even if you hold a PPL as the course is to teach you how to teach not how to fly. Your flying will be deconstructed and reconstructed - yes it was like learning to fly all over again 23 years after my initial PPL!
If you are really wanting to instruct then during the "hour building" phase please don't just bore holes in the sky. Go and do long cross countries, don't just follow the magenta line on the GPS. Go and do aerobatics, learn tailwheel flying, do an IR(R). It is all experience and will make you a more effective insturctor having some real world experience in your back pocket.
Having a PPL and FI will allow you to instruct the LAPL only. In order to instruct people for a PPL you also need CPL or ATPL theory exams passed. They'll take around 6-12 months of studying and passing of 13 exams. Now, this is what you need to decide, doing CPL exams means you'll need to do the ATPL exams at a later date for an airline career should you manage to get a Class 1 medical. So I would suggest going directly for the ATPL exams.
But. Here is a big but. The CPL and/or ATPL exams passes are valid for only 3 years to have a CPL issued. And to have your CPL issued you need the Class 1 Medical. If you don't manage to do that within three years then it'll be back to the books to pass all 13 ATPL exams again when you do obtain a Class 1 medical. In the three year validity you can get your self an SE-IR or ME-IR completed and issued using your PPL (you'll need an Audiogram added to your Class 2 medical) but the exams will expire in terms of CPL course/issuance if you don't have it issued within 3 years.
Personally due to having failed your Class 1 medical I would seriously suggest a non-aviation career (or a non-pilot aviation career) and instruct part time if you really want to do so and if you are able to obtain a Class 1 medical in the future then happy days. But this way you are edging your bets and establishing yourself in a career earlier rather than later.
Whatever you decide, I wish you all the best and hope it all works out for you.