How do I find out the reference genome for the UK Biobank? If you could direct me to an article that I could cite, that would be terrific.
I searched the official docs and found GRCh37 (hg19). Take a look:
The exomes are aligned to GRCh38 with the "chr" prefix removed from the sequence names.
The older SNP array genotypes, as Kevin noted, use GRCh37 as the reference.