If you only intend to instruct, you would not need to do the ATPL exams (14 in total) only the CPL exams (I think 9 in total, LASORS would give the answer)
You only need the ATPL exams if you one day wish to be an airline pilot, hence Airline Pilot Transport Licence. This actually requires a lot of multi crew time to get the full licence, which as an instructor you will never get.
The CPL exams can be done distance learning, and if you have the minumum hours you could then do the FI(A) course at a cost of around £7k. With this, you will be able to give ab-initio training, but not be paid.
If you do want to be paid you will need to do the full CPL(A) course, 25 hours in a complex aircraft at a cost of around £10k.
The other route would be a Class Rating Instructor (CRI) rating. You would not be able to give initial training, so no trial lessons or early lessons, but can give training to people who already have licences on things like Variable Pitch Props, retractable gear, tail wheel, single lever thrust and do people's bi-annual flights for revalidation.
If you have any thoughts of progressing to an airline job you would then need the ATPL exams and an IR, commonly called a "frozen ATPL", although you will not find this term in any legal document.