Only use the official site -
www.evisa.gov.trThe cost is $20 - roughly £15 - and the first question it asks is for your country of origin, where your passport was issued, such as UK. Once you have completed the process, you will be sent an email with your visa. It is linked electronically to your passport, so the chances are that you won't be asked to show it on arrival at Dalaman, but you should print off a copy anyway, as the law in Turkey states that you should carry a copy with your passport at all times.