First off HESA is not merely "some 3rd party statistics society", it is the recognised main source of data about higher education in the UK. And it gets its figures from the universities directly (with clear pro forma) - the differences between the HESA tables and the respective university publicity (Oxford is not the first institution where this query has been raised) are primarily down to how things are counted, including extra-mural students and whether part timers are counted per head or as a fraction (and more minorly over when in the year the figures are for). They're certainly not counting staff or academic departments. You may have a point about using a different HESA table but it should be standard across the board for UK universities for consistency and comparison - if we let each university article pick whichever table fits local impressions best then the information will be less useful overall.