The course content will vary from university to university. Retained A Level knowledge is useful, but not essential, as it is usually retaught at a higher level. Supplementary A Level refresher lectures are also sometimes provided.
In first year dentistry, we covered;
As aspects of cell biology - structure/function/signalling/mutation/technology
Physiology - cell physiology (action potentials etc), the cardiovascular system, the respiratory system, and the gastrointestinal system.
Anatomy of head and neck, and also of the heart, lungs, GI system, and brain.
Basic Introduction to Dentistry - including tooth structure and morphology.
Neurobiology.
Cell Biology and the Physiology courses are the ones covered briefly at A Level, however at university, they are taught from scratch again. Hope this helps!