Short answers, pulled from the story.
Greenwich Mean Time is the time system anchored by the Royal Observatory in Greenwich, London. It serves as the physical basis for a calculation that began as an astronomical measurement of local mean time.
Greenwich Mean Time was legally adopted throughout the island of Great Britain later in 1880 after being used by railway companies since 1847. A legal case in 1858 held local mean time to be official until this change occurred.
Coordinated Universal Time replaced Greenwich Mean Time because Earth's daily rotation has an irregular slowing trend making atomic clocks a more stable timebase. This new system is maintained by an ensemble of atomic clocks around the world starting from the 1st of January 1972.
Belgium sets legal time one hour ahead of Greenwich Mean Time while Ghana and Iceland use it as standard time all year round. Ireland defines Winter Time as the same as Greenwich Mean Time and Standard Time as one hour in advance.
The astronomical convention dating from Ptolemy referred to noon as zero hours while the civil convention dating from the Roman Empire refers to midnight as zero hours. The latter convention was adopted for astronomical purposes on and after the 1st of January 1925 creating a discontinuity of 12 hours.