The 80 ml bottle costs £55. That means that you have to pay £55 / 80 per ml of perfume, which is £0.6875.
On the other hand, the 50 ml bottles cost £45. Since it is buy one get one half off, you can do 1.5 * £45 to get the price of both bottles. That would be £67.5 for 100 ml of perfume. You would pay £67.5 / 100 per ml of perfume, which is £0.675.
It is cheaper to get two bottles of 50 ml perfume, so that is the best value for the money.
