As Haydn says, currently you don't need any qualification to own and use a boat up to 24mtrs in length,
other than a very deep pockets,

again only repeating what Haydn says, just google for "powerboat courses" there are plenty located along the south coast,

although my guess would be, that, without these courses under your belt, and some years of experience, no hire company is going to let you loose with thousands of pounds worth of motor boat,

to lease or rent is sound advice, did you get this advice from, Felix Dennis, "If it flies, floats or fornicates, always rent it.. it’s cheaper in the long run"

.

again google for powerboat leasing/renting, but you'll be faced with the same problem of no experience to show them

best of luck with your search.