To instruct at PPL level you will require 'Commercial level knowledge'. This means either holding a current CPL or ATPL if you wish to receive remuneration, or having passed the CPL exams if you do not.
If you have over 2000 Mil hours, your best bet would be to obtain your CPL (and preferably IR) under the Military Accreditation rights recently agreed. I'm sorry, but you'll have to do 5 JAR-FCL ATPL exams (rather than 14!) and then do your CPL Skill Test (and preferably IR) on something like a Seneca. That'll net you a 'JAR-FCL CPL/IR with ATPL knowledge', commonly referred to as a 'Frozen ATPL (fATPL)', with Multi-Engine Piston Class Rating - and if your Single Engine Piston Class Rating is still valid, that as well. If your revalidate or renew the IR within 5 years, your ATPL theory credits will then be extended accordingly; if not, you'll have to do ALL the ATPL exams again....
To add a FI rating to your licence, as an RAF QFI (but not a SEP QFI), you'd need to contact a suitable FTO for assessment, but I expect that you wouldn't need much more than a bit of groundschool and a FI Skill Test.
I know this all sounds rather OTT to many mil QFIs who just want to do a bit of part time puddlejumper instructing, I agree it is - but I'm afraid that's the way of the CAA..... Ask the Tornado mate on exchange with the Luftwaffe who flew over to the UK just for me to give him the RT exam! He scored 100%, unsurprisingly!
If you want to ask anything more about this, either PM me or e-mail me at my PPRuNe e-mail link and I'll try to answer your questions.