Why pilot or dental officer? The pilot part I can understand, but it's a pretty big leap from hoovering around at low level to hoovering around in someone's mouth - I'd make sure you have your answers squared away for that one.
As for do you need a degree to be a dental officer, I bloody well hope so! Remind me to check my dentist is actually qualified before she starts drilling for oil next time. I'm pretty sure that you have to be qualified before you join, but it should have told you that on the website. Incidentally, just a thought, do you not need chemistry to do medical / dental degrees? That's what I was told back in my day when I was doing A-Levels - if you want to do either of those, you need chemistry as medics and dentists are notoriously bad at teaching chemistry so they like you to have a good background to start with. Avoids embarrassing incidents like mixing up your gasses in the operating theatre because you can't tell your O2 from your NOx and wondering why the patient is going blue.
As for the rest, no you aren't tied down once you leave Cranwell, just don't expect much free time during training or if you end up on an ac type that is permanently deployed. PT, as long as you can pass your fatness test, no one will force you to do it, but it is a bit embarrassing when you end up having to walk to level 4 on the bleep test
Have a good rummage round the RAF website and use the search engine on here, plenty of good gen on careers, OASC and the like.