The flags are from the png100px folder at https://github.com/hampusborgos/country-flags with
GB renamed to UK. From the README.md:

This repository contains accurate renders of all the worlds flags in SVG and PNG format.

View all of the flags here: https://hampusborgos.github.io/country-flags/

We make every effort to maintain the most accurate repository of flags. That is, flags that most
closely match what the country or territory's laws and regulations describe the flags as being.

The source files were taken from Wikimedia Commons and sometimes contributed back (when the country
legislation more accurately describes what the flag should look like). The flags are not under
copyright protection since flags are in public domain (there may be other restrictions on how the
flag can be used though).
