When I went for my 1st Class Medical, I took my current presciption with me and wore my contact lenses and had my glasses in my pocket.
I had to take my lenses out and they tested my eyes "un-corrected." I couldn't see anything when I took them out, and the bloke asked how far down the board I could read before it went blurry. I replied "wheres the board gone!!"
I dont think its necessary to have your prescription unless you have corrective lenses, but best check before you go!!
I passed my medical too!!
G-ANDY