You're going along the right lines, I'll detail as much as possible!
Yes, you will need to be an RYA PBI, and with your experience that should be as simple as a 3 day course run by an RYA Trainer, and then Moderated by another Trainer. Plenty of centres run these, check out Plas Menai, Calshot, Stormforce, Griffin Marine Services, Powerboat Training UK, SWAC, loads of them about! You would have to check your St Johns First Aid Cert is HSE approved and includes treatment of Hyperthermia and Hypothermia.
After that you will need to apply for RYA Recognition as a training centre. This involves satisfying the RYA that either the Principal or the Chief Instructor holds the relevant qualifications, boats are suitable and insured for training, and the centre is suitably equipped. They will also check risk assessments, standard operating procedures, and if you wish to teach Children a Child Safety Policy.
They charge an inspection fee, and an annual recognition fee. Things they check are minimum level of kit for the boat (Flares, foot pump, VHF, Anchor, toolkit, fire extinguisher, paddles, that sort of thing) as well as general seaworthiness, adequate classroom facilities, and adequate changing/shower facilities. The boat does not have to be MCA coded.
The inspection takes a day (although a fair bit longer to get a date for and to prepare all the paperwork for). You could either gain recognition as a centre in your own right, assuming you have the facilities available to you, or as the British Divers Marine Life Rescue training centre.
Info on this can be found here http://www.rya.org.uk/WorkingWithUs/...dpwcourses.htm
You cannot instruct as just a PBI + Boat, you do have to be attached to a centre of some description. It is possible for existing centers to have your site added to their recognition.
Hope that helps, any more q's and I'll try and answer them.