UK list is embedded (authoritative Bank of England lists). US list is populated from the FDIC bulk CSV at runtime (live/full FDIC institution list). See notes below about sources & CORS.
Source for UK list: Bank of England — PRA/BoE compiled lists (bank/brand lists & PRA-regulated CSVs). See citations below.
The US select is filled at runtime from the FDIC bulk data (Institutions CSV / API). If your browser blocks the request due to CORS, use the manual fallback button.