The PGDL takes one academic year. It is not offered by the University of Oxford, but you could take the course at Oxford Brookes, and several other places.
To obtain a PGDL you must pass exams in -
Public Law (Constitutional and Administrative Law)
Criminal Law
Contract Law
Tort Law
Equity and Trusts
Land Law
EU Law
You could also do a two year Senior Status law degree at Cambridge and a few other universities.
Lots of practising lawyers have degrees in history. I am one of them. The Oxford approach to history is a great foundation for practising law. If you don't go into the law, you could seek a career in more or less any thing which does not require STEM skills.