Publicly funded educational institutions normally charge two levels of fee: a lower 'home' fee and a higher 'overseas' fee. Private sector institutions often have only one level of tuition fee, which all students must pay. Whether you pay a 'home' or 'overseas' fee depends on whether you meet certain criteria.
The fee status criteria is provided in regulations, and guidance, published by the governments of the four countries of the UK. Different criteria will be applied depending on which country you are studying in, and whether you are studying a course at higher education (HE) or further education (FE) level.