I'm not clear on what level of "pretty low" I'd have to see before being reassured here. I guess it's HN, where I'm the only dummy who doesn't know what acceptable changes in everyday metrics like "Parahippocampal gyrus thickness" and "Lateral orbitofrontal cortex volume" are.

Your second remark is also peculiar: the "obvious sampling problem" to which you allude is either (1) a subtler-than-expected-point about the UK Biobank's program (designed to be a mass participation program to observe longitudinal effects in previous healthy or as yet undiagnosed people) or (2) evidence you are just making stuff up.