Context: I'm a heavy guy - at my widest point, about 58 inches (147 cm) around, and one of the ways I exercise is to incorporate it into some other activity (walking or cycling to the shops, commuting by bike to work, walking where practical while on holiday). I live in the east of England, so recent experience of hill country is limited, but I have done some hiking as a child/teenager in the Lake District and on Dartmoor/Exmoor while at University.
I enjoy camping and have successfully done some bikepacking between commercial campsites, so am interested in backpacking/camping. Based on my experience bikepacking and various instructional videos, I expect to need about a backpack of about 60 litres capacity for a multi-day hike of 10-15 miles per day in moderate country (liable to increase as my confidence grows or if weight starts coming off).
Unfortunately, the backpacks I've tried in the past tend to be optimized for someone much slimmer than me, which becomes noticeable in inadequate length of the chest strap and (particularly) the waist belt. In some cases, when carrying a loaded day bag, the length of the shoulder straps has also been a problem, as they can cut off circulation to the arms over time. A less important but still 'nice to have' thing is good airflow - unsurprisingly, I sweat more heavily than most, so I have been impressed with some suspension-style backpacks.
Can anyone recommend backpacks or manufacturers that cater for the fatter hiker? Spending is not a particular issue, but availability in the UK may be (I can import, but it would impact the ability to try before buying).