The Encyclopedia Galactica is derived from the real Encyclopædia Britannica, the oldest English language encyclopaedia still in print. The current, 15th edition of the Encyclopædia Britannica consists of thirty volumes. There is also an online edition, Britannica.com.
Although Douglas Adams claimed that “a number of environmental and weapons treaties and laws of physics” prevent the mixing of a Pan Galactic Gargle Blaster on Earth, fans have come up with several recipes. Some examples can be found on the bar-tending wiki.
Reader's Digest is an American family-orientated general interest magazine, first published in the 1920s and still going today.
All of the places named here are satellite earth stations monitoring space, particularly looking for evidence of extra-terrestrial life. Appropriately enough, at Goonhilly the world's first parabolic communications antennae was dubbed 'Arthur', although this was for the mythical British king of Camelot, not Arthur Dent.
Marks and Spencer is a quintessentially middle-class department store, commonly found on the British high street. It is traditionally renowned for luxury groceries; plain if reliable underwear and women’s clothing; and good quality household linen.
The Visigoths were part of the Goth tribe and famously sacked Rome in 410CE.
Godspell and Joseph and the Amazing Technicolor Dreamcoat were popular musicals from the 70s. As Ford Prefect pretended he was an out-of-work actor for the duration of his stay on Earth, copies of either the albums or scripts of these musicals could be part of his disguise.
Listen on Spotify:
Hyperspace is a very common sci-fi trope, a technology that allows spacecraft to travel faster than the speed of light. It may more acurately be called Minkowski Space.
Basingstoke is a town in a southern England, nicknamed 'doughnut city' because it has more than ten roundabouts on its ringroad.
Barnard's star is a red dwarf star six light years away from earth, named after E E Barnard who discovered it had a very large proper motion in relative to other stars.