The Lowepro Nature Trekker is listed as being airline carry on compatible on their website. Has any one any experience of taking it on a flight with a UK airline?
You'll be OK with the major airlines (so long as you don't fill it to bulging! ) although it is a weeny bit bigger than the actual allowance in one dimension.
But do check the airline websites for up top date info if you're thinking of flying RyanAir or EasyJet or any of the other cheapies: their allowances are less generous in terms of both dimensions and weight.
I have a Photo Trekker AW and have flown on US domestic flights with it as my carry-on numerous times with no problems at all, and I believe the Photo Trekker is a bit larger than the Nature Trekker, but check with the individual airline to make sure.