Thanks for the replies guys, I'm not using alice packs or karrimor as an option because looking non-military is quite an important specification for me, I don't want to stand out in that way as much as possible. For that reason I've crossed off quite a few quality makes - maxpedition, 5.11 tactical etc. But this spec is important for me, OD green or camo with lots of molle looks way too tactical - even though I do like it and think it looks awesome.
Mesh stow pockets on the sides are indeed a great future. From what I can read only the Camelbak has front webbing. Make sure you validate the quality of the zippers too, often one of the weakpoints for these types of bags because they get to endure a lot of stress.
I'm a bit surprised why you want two main compartments (three if you count the bladder compartment). I would suppose multiple main compartments are mostly useful for packing clothes and traveling, as opposed to active use. Do you mind elaborating here?
Otherwise I may point you in the direction of Sandpiper of California (US - their Bugout Bag is a classic) and Karrimor SF with the Delta range (UK - you can opt for black instead of olive). None of these bags tick all boxes though but since you've done some serious investigation I'm sure you'll appreciate some of these interesting brands.
Sorry, I may have used incorrect terminology. All four bags I've shown have what I called "front webbing", also seen it called daisy chain or front looping. I want two main compartments to have the ability to separate and sort gear somewhat - I find having one main large pocket with
everything in can become tedious rather quickly. When it comes to gear I'm a pretty huge organisational freak and the more pockets for organisation the better - within reason!