What is the British word for auditing?

I just checked online, to audit as a verb in AmE means to attend a class without being tested at the end of semester, only going to the class informally. I wonder which word do English people use?

The British don't use "audit" in that sense (listening to academic lectures without doing assignments or taking exams) nearly as much as North Americans. In most British universities all lectures are open to all students of the university, so you can "audit" a course simply by turning up at the lectures whenever you feel like it. Therefore it doesn't really need a special word.

The best equivalent is to "sit in". For example

I sat in on Professor Smith's lectures on nineteenth century realism.