At 15 you might find it hard to get RYA clubs to use you for safety boat work. Your RYA PB2 will be endorsed something to the effect of "under adult supervision". You will also find some clubs automatically suspicious of anyone who just turns up wanting to drive powerboats (that may be a sign the club is full of real-WAFIs so an early warning to avoid them!).
You wouldn't normally be expected to have "life saving" (as in Bronze Medallion or whatever it is called these days) to do this sort of stuff, but an RYA First Aid at Sea certificate would add credibility (as would the VHF cert - if you are old enough?). Likewise with the safety boat certificate but I suspect that you need to be 16?
You will however spend a lot of the time pottering, or floating doing nothing. Good for close quarters skill development but you can spend 100s of hrs doing safety boat work in a small area and never develop your longer distance cruising, rough water handling, passage planning skills. It can be terminally boring, and not as much flying about in a rib as you possibly imagine (especially not if you want invited back!).
No idea about dive clubs but would imagine similar principles apply.
There are some "clubs" that share ribs and use them, but ignoring your age which is probably a problem there, I think I can discount them for you on the basis that they are hideously expensive (you could buy a cheap used rib for the annual fee) and will be big engines that cost many £ to refill!
Venturers Rescue is based in Hampshire (New Forest). Its a bit of an odd organisation from what I can see - and you'd need to go and see if you would fit in (they seem to be a "uniformed" organisation so will be quite structured and formal - I've no idea how someone with a bit of experience turning up will go down - but its potentially a very low cost entry route). Welcome to The League of Venturers Search and Rescue web site
GAFIRS who C2 mentions do specifically do "youth work", but it looks like it is canoe/surf lifeguard type work rather than ribs: GAFIRS - Cadets
Junior Ribsters might be of interest but I think they are based in Wales so not exactly handy - Junior RIBsters
Finally I don't know if any of the powerboat schools round your way have any specific programmes for "youth" - I know that at least one of the Scottish ones was running activities centred around the Honda Junior Rib challenge (or whatever it is called) a few years ago.