Visas in Turkey have changed to what they used to be, you used to just arrive in to Turkey and go to the visa counter sand purchase a £10 visa before going to passport control. Now they have brought in the e-visa which you buy online for currently $20 USD (this changes a lot, so check on the website). This process takes about 10 minutes. British passport holders need a visa to enter Turkey.
You can apply for a visa up to 3 months before you travel, your visa is valid for 6 months and you can stay up to 90 days in a 180 day period.
The website for e-visas is: https://www.evisa.gov.tr/en/
Don’t panic if you do forget to obtain a visa before you travel, if this happens to you, when you land in Turkey head over to the visa counter to obtain a £20 visa. although i would always advise you to obtain one online.
If you do not hold a British passport or you are travelling with residency permits, please visit this website for more information on entry requirements, this website is always up to date and very helpful. https://www.gov.uk/foreign-travel-advice/turkey/entry-requirements