Two real options - new or fleabay. Fleabay will give you the big favourites, Berghaus Munro or Karrimor Sabre or Patrol, you’re dependant on what’s about, but if you can wait you’ll find a spectacular bargain.
HM supplies do their own 45ltr bag at £55, by all accounts it’s very good, and Snugpak do a 40ltr bag called the Stamina which is pretty well made.
If you’re staff I’d head more towards the 40/45ltr end of the market, if a cadet then 30/35ltr will happily do you.
The old favourites - the Berghaus, the older karrimor - will last a lifetime, you can buy a 20yo one on fleabay and it will last you another 20.
Personally I wouldn’t touch the Highlanders, the webtexs, the Kombats etc… I’ve seen then fall apart on their first weekend outing, and they won’t cost you much less than a decent sack.