The GDL is a professional course which is compulsory for those who want to be barristers and do not have an LLB and is preferred for solicitors who do not have an LLB. As far as I am aware you have to have a degree first to be eligible for the course because of the professional and regulatory requirements for solicitors and barristers. 80% go to BPP or university of law for law post grad so it may be worth checking on BPP's website or that of the Central Applications Board - the CAB is a kind of UCAS for lawyers for those applying for courses like the GDL, SQE courses, bar course etc.