There is licence free radio in the form of Citizens Band and PMR446 however if you wish to legally use more power and have access to other frequencies and modes as well as the world-wide network of operators you need to obtain an amateur radio licence.
The current licensing system as administered by Ofcom is tiered and consists of three levels Foundation (M6), Intermediate (2E0) and the Full/Advanced (M0) Licence.
You take the Foundation first followed by the Intermediate and then the Advanced once you have passed both the others. Progression through each stage is not compulsory – you may stay at whatever stage you wish and enjoy the privileges that licence offers.
The Foundation and Intermediate consists of short exams and practical assessments and the Advanced is a longer exam, each stage requires more knowledge and offers greater privileges such as increased power, bands and methods of operating. But don’t be put off by the thought of the exams the Foundation and Intermediate exams and practicals are straightforward.
The following video by Essex Ham gives an introduction to Amateur Radio and the Foundation licence.