What is a 'Chair' in British academia?
It is the difference between the role or job and the person who is doing that job.
The person has achieved the status and title of Professor. That person can take the position of the &quot;Chair of Computer Science at the University of Somewhere&quot;.
Sometimes a particular Chair will have been endowed or funded at a University and then a Professor is recruited to take that Chair. That  person could have  moved from another Chair at some other University.
Some of  the Chair endowments are quite old, like this example from 1702 in Cambridge: https://www.ch.cam.ac.uk/alumni/new-name-our-oldest-chair