What does 'to mark' mean in the UK? It is just to evaluate what is right and wrong in a script (i.e., an exam or problem sheet completed by a student), and making notes and comments on that?
To mark is to evaluate student work based on the rubric established for that work which, itself, will be a combination of University policy, subject policy, and what has been presented in the course administrative documents. 

If you are unsure of what is expected of you with marking, it would be wise to discuss this with your head of subjects or course coordinator as you will need to conform to your university's policy on marking and could face issues if your marking deviates from that.