Possibly Reader, but hopefully professor since (posted) Reader salaries aren't great.
BTW, I've seen "Chair" as a rank, and it's differentiated from Head of Department... does Chair always imply admin duties, like it might in the U.S.?
Also, is it true that Oxford has dumped in-between ranks? I see Lecturer (with wide pay range) and Professor, with nothing in between, at least where I looked.
Chair = professor. Head of Department is something else entirely.
At most universities professors are expected to take a share in administration, just like everyone else, and as a professor you are usually expected to take a more senior role in a department/university eg director of research, director of teaching.
I know a number of universities that no longer have the rank of Reader. Readers are often (always?) paid on the Senior Lecturer (Principal Lecturer at a post-92) scale so there is no salary advantage over SL.
Also, do you really mean 'impact' when talking about the REF? For the REF it has a very particular meaning, which is not necessarily directly related to publications.