Citizens of United Kingdom can enter Lesotho visa free for up to 14 Days. You only need to purchase a visa if you intend to travel to Lesotho for longer than 14 Days.
British nations can obtain a 14 day visa on arrival. If you wish to stay in Lesotho for longer then you must obtain a visa beforehand.
Your passport must be valid for at least 90 days from the date of your exit from Lesotho, and you must have at least 2 blank pages.