For thousands of years, timekeeping was kind of a loose thing. When the fastest anything can travel is on horseback, exact times aren’t, strictly speaking, necessary. So in England, as in most places across the globe, someone in a village or town – the local priest maybe, or a prominent citizen – would use simple instruments to figure out when the sun was directly overhead (a.k.a. “noon”) and set up a sundial or clock accordingly. And the rest of the town would be synced to that. And if it wasn’t 100% accurate, or if the village 5 miles away decided to make their clocks an hour earlier – or 20 minutes earlier, or 14 minutes later, or whatever… it just didn’t matter all that much.
Then one day the railways came, and it was nigh impossible to schedule trains using every tiny village and town’s version of “local time”. Thus, the concepts of “standard time” and “time zones” were born. And then the Statutes (Definition of Time) Act 1880 was passed by Parliament, making Greenwich Mean Time the one official time on the island of Great Britain:
Whenever any expression of time occurs in any Act of Parliament, deed, or other legal instrument, the time referred shall, unless it is otherwise specifically stated, be held in the case of Great Britain to be Greenwich mean time, and in the case of Ireland, Dublin mean time.
Because the Channel Islands are not part of the United Kingdom, GMT wasn’t adopted in the Isle of Man until 1883, Jersey in 1898 and Guernsey in 1913. Ireland, then a British colony, didn’t switch until 1916.
Interestingly, there are a handful of public clocks built in Britain during the transition period between the two systems. The clock pictured behind Victoria and Jimmy on this QI screencap is in Bristol. It has two minute hands. The black hand is for Greenwich Mean Time. The red hand is for the local “Bristol Time”, which was 10 minutes behind GMT: