I can not answer with an 3xplan@tion, as it is not PC, a joke!
Backpack: I do not know how the British Armed Forces units do it, but in the Swedish AF, some units , the units that are issued the various sized backpacks had a very specific equipment list and an exact way how to pack. You could add, but not remove, any equipment.
But if you added anything, you had to place it so the specified items were at the specified place.
The short mission pack, Ryggsäck 35L Komplett, what you guys call Lk35, was enough for around 4 days except winter.
Remember though, our battle uniform trousers had two large thigh pockets, two large front, two large back, the tunic two large front, two breast and two large in the back.
I still remember - 1:st Aid, autoinjector box, leather thong ( to stop heavy bleeding) left thigh.
The content of the other pockets bust have been around 4 litres or so.
To be frank, a well designed 35 l’iter backpack should be enough for three days.
What you need to do is to minimise and pack outside the backpack