The foundation year dental course is competitive and you really will have to be very motivated to get into it, lots and lots of work experience etc and lots of extracurricular activities. You would have to have a very good reason why you didn't apply for dentistry first. You said the reason you carried on with biol is because you were always interested in dentistry, so interested in fact you didn't check the entry requirements, I know people you have had no biol a-level but have chem so can do dentistry, I can only speak for Bristol but you definately need chem, the course involves so much in the 1st year, you'd be so stuck without it! But as you said there is the foundation year of course. The fact you re-sat will put you at a very big disadvantage, they are really not going to look very kindly on you failing from sheer laziness as dentistry is an intensive and hard course and it is very expensive to train a dentist, if they think there is a chance you could get caught up in the whole student fresher thing and do no work then they will never accept you.
If there isn't anything which interests you at uni, I'd say do as well as you can in your a-levels, take a year to think about what you'd like to do with your life, look at what you need to do and work towards it. Do some volunteer work, lots of work experience in a few different things, travel etc and hopefully you should be able to decide if you need to go to uni or get a job to get to your target!
Good luck!