It isnt just that its 'competitive' - just remember that Grad Entry Dentistry is still an undergrad degree, and that you will not get Student Finance if you already have a 4-year MPharm degree.
If you want to do Dentistry, do Dentistry, not Pharmacy.
And if you are not sure, then plan a gap year (or several) and get some relevant experience in each work area - then decide what you want to do.