NavList:

Hello Rafael,

Good to hear from you! :) You wrote:
"the GPO website states that "the book is produced in collaboration with His Majesty's Nautical Almanac Office in the UK". I seem to recall that Frank Reed once mentioned in a NavList post that all data for this almanac are generated by the UK Nautical Almanac Office at this time."

As I understand it, HMNAO produces the print-ready PDF or some precursor document that is nearly print-ready. They also do many of the final coordinate transformations that put the data into the format required by navigation, like GHA. The primary source for the ephemeris data for the Sun, Moon, and planets is still the NASA/JPL numerical integrations. Up to here, my information is based on personal communication with people directly involved in the production of the Nautical Almanac.

More speculatively... I don't have any specific information on the star data, but this is almost certainly Hipparcos data (European Space Agency, thirty years stable) with calculations for coordinates "of date", including proper motion, precession/nutation, aberration, produced in-house by HMNAO. Although GAIA mission data (also ESA) for the stars is radically more accurate, the bright stars on the celestial navigation list were specifically avoided by GAIA. And in any case, the higher accuracy of the GAIA data would never matter at the tenth of a minute of arc precision of the Nautical Almanac.

Finally, I have learned nothing new on the production of the Nautical Almanac since 2017, and it is possible that my information is out-of-date. But it is certainly true, as it has been since I first started saying this at least fifteen years ago, that anyone today with good knowledge of positional astronomy could reproduce a near-exact equivalent of the Nautical Almanac from scratch. There's no magic.

Frank Reed