I never have seen a rucksack with internal frame where the belt was attached to the frame. Usually you can easily pull the frame out after opening just one strap buckle.
I don't know the Sabre 75. Theoretically it could be too large for you. You aren't the tallest, but also not really a dwarf. I think it should fit you, but I stay unsure in this case. It is possible that your carrying system is too large.
I recommend to compare the dimensions of the carrying system of Sabre 75 and 45. Are they identic or is the carrying system of the Sabre 75 significantly larger?
If yes, it's perhaps too wide for you.
If not it seems to be a packing fault.
Unfortunately
@eraaij didn't yet tell us how tall he is. But he could have had the problem into the other direction. A too tall Dutch in a too small British rucksack???
Berghaus offers the larger - non adjustable - rucksacks for several back lengths, like clothing is offered in several body sizes.
One size fits all doesn't work with non adjustable rucksacks with hip belt.
If the rucksack carrying system isn't adjustable it fits correctly only people with approximately the body hight that the makers did choose for the average man.
And it is absolutely possible, that the perfectly fitting back length of the smaller packs is smaller than the perfectly fitting back length of the larger pack.
A very small person usually has problems to find a well fitting large rucksack, and a very tall person usually doesn't find so easily a well fitting small rucksack.
It's obvious, that the makers assume that a small guy can't carry a fully loaded 120 litres rucksack anyway. And they don't offer such constructions for the rare exceptions.
I mean, generally one needs to keep that in mind. But otherwise you seem to look pretty normal.
I think you should try to pack it differently.
And you should write an e- email to Karrimor SF. They seem to have an excellent client service and they also can tell you if the rucksack was constructed for your back length or not.