FWIW I have a
Deuter Race EXP air which I'm very happy with. It has a mesh which eits on your back with a gap between the mesh and the rucksack back which means far less of a sweaty back.
If you're doing LEJOG, might I suggest ditching the rucksack idea and seeing how much you can fit on your bike. It will be much more comfortable. Get a front (and possibly rear) triangle bag for the frame, a big saddle bag (or get a seatpost rack) and, if needed, a handlebar bag.
I travelled back from edinburgh to manchester over 3 days using my rear cycle pockets, a front triangle bag and a big saddle bag. I took a spare pair of duds, a toothbrush and paste, a pair of shorts, a cycling top and my general cycling stuff (tools etc). I washed my kit each night in the b&b's.