If, when you take your first medical for flying, the Doctor decides that you need to wear glasses for flying, then he will mark your medical certificate with the restriction that says just that. In addition you will be required to carry a spare pair.
Legally you could possibly argue that your prescription sunglasses are your spare pair, however, if you really need to use the spair pair and the light is fading, you could regret the decision not to buy a spare normal pair.
If you are about to invest several thousand pounds on a PPL, why not invest in a cheap pair of glasses to keep with you when flying?