To be allowed to transmit as a radio amateur you are required to have a licence.  This is a legal requirement that applies to all countries not just the UK.

Amateur radio in the UK operates a three level licence structure:  Foundation, Intermediate and Advanced.

Foundation Licence = 10 watts.  Call signs M6XXX / M7XXX
Intermediate Licence = 50 watts.  Call signs 2E0XXX
Advanced Licence = 400 watts.  Call signs M0XXX

Each level of licence allows you to use differing levels of transmit power (see above) and each licence offers other slightly different benefits.

Each licence requires you to pass an exam.  But don't worry, FARS have accredited teachers and examiners to help train students in all aspects to enable them to pass the exams.  A £10.00 fee is charged for all courses and exams (reduced fee for under 18's) - included is FREE membership to Furness Amateur Radio Society (which runs until November of the year taken out).  No fees for FARS members.  NB - exam fees also apply.