MUSTFLYWILLY
If you can find a non profit driven club that is actually run by members for members then why not work for free. Please don't do it for any club that charges customers for your time and then pockets the money themselves (illegal and unethical)
You will need to pay for your own license renewals and medicals so please factor that in (you may wan't to charge for "expenses").
You mention that you are in the Navy so I would suggest you check out Navy flying clubs or failing that RAF flying clubs. You might like to know that the it is possible to become an RAF Instructor where your customers are air cadets, you do not need a FI rating and can instruct on a PPL (unrenumerated of course) - I didn't know that but if I had I would have investigated opportunities locally.
If you go the FI route you will need to pass the CPL exams and if you do that it would then make sense to complete the CPL training (but it would be more sensible to bite the bullet and sit ATPL exams as they offer more opportunity for career advancement). Either way you would then most likely wish to be paid.
Other countries have non renumerated instructors but they are non profit driven real members clubs. Lets face it if you can log hours for free, avoid expensive and unnecessary extra training and help somebody else at the same time that is a real win win (JUST NOT SO COMMON IN UK THESE DAYS).