Might not be big enough, but how about this?
Large Molle 3-Day Assault Tactical Military Style Backpack
£33.24 + £1.99 P&P
It is dispatched from Hong Kong. so it could take up to a month to arrive and you may have some questions over the quality of this bag, but I drunkenly purchased this Maxpedition versipack copy from the same supplier:
Molle Tactical Utility Shoulder Bag
£13.29 Free P&P
... and I have to say, when it turned up (and as I'd completely forgotten I bought it, it was a bit of a surprise when it did) I was absolutely made up with it. Well made and very handy.
Might not be for you, but if you are looking, worth a gander.