Hi Pablo1985
I recently renewed my Class 1 in Gatwick and i wear glasses. Don't quote me on how it works as i can't be totally sure, however when i went the doctor put me in front of the chart without my glasses initially and needless to say, it was a little tough. He then said to put my glasses on and give it another go, and i read my way through the whole chart. He was perfectly happy with it.
As someone who wears corrective lenses i can understand your concerns, and although my prescription is quite mild, the same fears always crop up in the back of my mind. Upon completion of the test i asked him what tolerances i still had left with regard to the eye test, and he just said 'i wouldn't worry, you have plenty to go at'.
I would imagine they'll test you without first and then test you with lenses after. As mine was a renewal i can't be sure if it's the same if you're going for an initial medical. My advice would be to get aa copy of the testing standards from the CAA website, give them to your local optician and have an eye test before you go.