If it's a non-refundable deposit, you probably won't get it back.
If your goal is to pursue a career in medicine and you can afford the fees of the MBBS programme you've gotten into it (and there aren't any concerns with recognition by the GMC if you also want to practice medicine in the UK) then that might be the better option.
That said, knowing Buckingham is a private medical course in the UK, you could always tell them you have an offer for a full MBBS elsewhere when asking after the deposit for the CertHE. Maybe they'll find a place for you on their main MBBS if faced with losing the tuition fees from the CertHE offer...although that would really be a damning indictment of their business model I think if it did happen.