Basic entrance requirements for admission to doctoral programs in most British universities are:
For admission to the teaching and research programmes usually requires a master's degree in the same or a related field, and CV, confirming the experience of scientific research and independent research work.
On most professionally-oriented programs having a professional experience is an entry requirement.
For admission to research doctoral programs is key research project (Research proposal).
All universities set their own requirements for level of English and valid certificates. Detailed information on English language tests and preparations You will find here.