Most degrees for dentistry will require both biology and chemistry, so you're severely cutting your chances thin especially for an ultra competitive degree.
Thank you for responding I would take biology but got a B instead of an A meaning I wouldn’t be able to take it Shall I still do it privately
Shall I still do it privately I would recommend it if you want to increase the scope of unis that you can apply to.
However, I would also check to see if you need to do it all in one sitting - see the entry requirements of individual degree to be sure. If so, you would need to do this alongside your current A Levels as opposed to during a gap year.
Also doing Biology with the practical assessment (if it's required for particular degrees - which should be most of them) is a complete pain. There are only a handful of exam centres that would do practical assessments, and they tend to be expensive (£1000 for example for the assessment alone) and follow a different schedule (which will vary depending on the exam centre) compared to the exams (May).