"…personally, have no experience at all in what Multi is trying to do…"
I do.
I walked about 250 km in the Cévennes carrying a Sabre 45, varied terrain, lots of rough stony paths and a few climbs. I also carried it on a trek across the Fogaras Mountains in Transylvania, I'm not sure how long that trip was.
I chose the Sabre for the Cévennes trip because I planned to do some canoeing at the end of the trip and the sabre and its side pockets break down and fit into drybags or drums more readily than some of my other packs. I chose it for the Transylvania trip because I thought at the time I'd be spending a little more time in the forested lower slopes and not walking across the roof of that particular part of the world.
Although it can be nice to organise your gear into the three seperate compartments, the weight of the pack is an issue, when walking long distances day after day that weight will grind you down. Small injuries will not repair and perhaps more importantly you'll spend more time looking at the ground in front of you rather than enjoying the enviroment that surrounds you and all that there is to see.
The pack is comfortable enough for weekend trip, maybe a little more, for a long journey as the OP suggests he has planned, people should look elsewhere.
A warbelt?
Give your head a shake, you are not tabbing to Port Stanley, you are not setting out across Roger Zelazny's Damnation Alley, you are planning a long walk around the British Isles with the odd detour for Hadrian's wall and such. You need a good pack, designed for distance walking, you need a nice pair of boots, you need to step away from the computer screen and get out and about and meet some of the folks who live on that island with you, with very few exceptions they are nice people, they will be fascinated and entertained by the adventure you are embarking on, many will go out of their way to help you in any way they can. Which I hope you will come to appreciate if you ever manage to get north of the border.