What is "Honorary Lecturer" in the UK universities?
In this link they explain the subject in detail
In general regarding honorary academic titles for non-employees it is written:


  To recognise the association, the College may confer an honorary academic title on a non-employee during the period of their association.... The title to be conferred will depend on the level of distinction and qualification of the candidate.


And specifically with regard to "honorary lecturer":


  Honorary Lecturer: Must continue in teaching, research or joint industry/academic activity to an appropriately high level