Due to the competitiveness of medicine and dentistry, they are not in clearing. Medical schools get thousands of applicants for a couple hundred places each, and so all their offers are given out.
As for applying to medicine and dentistry, they're not easy subjects. If you're not passionate about them, and willing to dedicate pretty much the rest of your life to learning (in medicine's case), and very few hours' sleep a night, then there's no point applying. People drop out of medical schools. It's not easy.
But, if you're only doubting your academic abilities, and your "slim chance" is only because of a poor UKCAT/BMAT grades, then go for a university that will focus mainly on good GCSE's or A levels. You need AAA for medicine, realistically, and if you're predicted those grades, and really work hard to get them, then there's no point not applying to medicine if that's what you want to do.