They looked at geographic structure and found that the UK Biobank is subject to a lot of stratification in that regard. They looked at BMI (body mass index), household income, and educational attainment and found all of them to be subject to geographic population stratification, even with principle component analysis. First they looked at a smaller subset of genetic data from a previous study (ALSPAC)
...we anticipate that the educational attainment of people who migrate for economic reasons differs from people who do not. Educational attainment is therefore aligned to subtle genetic differences even in this apparently geographically and ethnically homogenous population and this is co-incident with axes of ancestry.They move on to the beast, the UK Biobank:
