like the previous post mentioned, there are foundation courses, of course that means taking a year out to reapply. As for re doing a levels, you should check with universities whether they allow it since some want you to do a levels in 2 years, not sure whether it would count for 2 years or 4 years for you...
And lastly, I think you should research REALLY well what the jobs involve... The fact that this seems to be a last minute decision and that you're undecided between medicine and dentistry shows that you don't actually know much about the jobs since they are VERY different from each other. So you need to be truthful, are you looking to go into these professions for the job security/money/prestige or do you actually want to be a doctor/dentist, becuase its a loooong road.
The other alternative is to finish your english degree and apply for graduate entry.