I went to Turkey in May 2013 on my US passport. I'm uncertain as to whether or not anything has changed since that time, but at the time I had to purchase the visa just before I got to the border in the airport. There was a desk right before I went through immigration. With my US passport, I had to pay $20 in US dollars. The woman stuck a sticker in my passport which was the tourist 'visa', valid for 90 days. Apparently for UK passports, it is £10, which has to be paid in pounds sterling. So in addition to getting money exchanged to Turkish Lira, I had to ensure that I had 20 USD as well for the trip.
Hope this helps!