Berghaus Munro. Used by german army mountain troops. You can get side pouches for it.
Older model was the Bundeswehr Gebirgsjägerrucksack 34 Liter, Cordura.
That weights only 775g, the foldable german army sleeping mat, 425g is used in there as an internal frame. (Volume and weight the following shop tell you are wrong, but it is the right ruck sack.)
https://www.raeer.com/shopexd.asp?id=17838
Older canvas versions of that rucksack are heavier.
The Norwegian Army also used a cool old school ruck sack, made from Nylon or before it canvas.
https://www.raeer.com/shopexd.asp?id=17527
Berghaus Centurio could be an interesting option, here you could change the side pouches containing special stuff for special use.
ESSL Kaderrucksack, used by the Austrian army. 34 Litres or 40 litres for taller people.
Good and cheap!
The prices shown here mean new stuff:
https://www.essl-rucksack.at/produkt/essl-ru502-rucksack-34-liter-oliv/
Decathlon hunting ruck sack 30 litres
https://www.decathlon.com/collectio...hunting-backpack-30l?variantid=1128963571725#
(Sorry, I can't give you the link to the British shop, surch it yourself, please)
Together.with two X-acces organiser pockets SMALL as side pouches.
The 50 litres version weights the same: ONLY ONE KILO!
Or the heavier Swedish army pack, of course.