The RYA can supply you with a Principal's Pack, and Guidelines for Centre Inspections. Between those 2 you should have all the information you need to put together an Operating Manual.
Personally, I think you'd be better to start from scratch and produce your own rather than grab ideas from others. The Operating Manual defines the way you run your business, and it should reflect your own school which will be different from everyone else's. I inspect RYA training centres, and I don't think I've come across 2 Ops Manuals that are the same - but each one is effective for the school that produced it.
One tip - make sure your Risk Assessment is thorough, a lot of the rest starts to fall into place once you understand the risks you have to deal with and create the procedures to contain those risks.
Good luck with it.