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I think all MINIs comes standard with a 12 yr/unlimited mileage warranty against rust and any coorosion. I don't think its necessary to rustproof if you get that kind of warranty.
This is a modern car. No need for aftermarket rustproofing. MINI is got you cover for 12 years/unlimited mileage for the very unlikely event of body rust.
Also, depending on what the aftermarket rustproofing process entails, it may actually void the manufacturer's rust warranty (I'm thinking specifically of the old Ziebart method where they had to drill holes in some of the factory sheetmetal to spray the rustproofing solution behind the panels). I don't know if rustproofing companies still do it this way, though - my last experience with Ziebart was 25 years ago.