In Buffy the Vampire Slayer, was there ever any reason given for all of the Watchers being English? Or does the show just make them such because the accent is cool?
The Watchers' Council of Britain originally began as the Shadow Men (creators of the Slayer) in ancient Africa. As far as I know, no canon stories have detailed the history of how they came to settle in Britain.
Being based in Britain, it makes sense that the majority of Watchers would also be British. This is especially true in modern times, as we see that there is even a nepotism aspect with Watcher families like the Giles and Wyndam-Pryces.
Not all Watchers are British (although most that we see are). For example, Merrick, Buffy's first Watcher was American (both in the non-canon film and the TV series). Robson is played by an American actor (I can't recall if he used a British accent in the show or not). Kendra's Watcher, Sam Zabuto, may not have been British (his name hints at his being Jamaican like Kendra), but we don't see him on screen or in the comics.
Andrew Wells, an American, appears in two episodes of Angel season five and is being trained as a Watcher. In Season Eight of Buffy:
Andrew is seen leading a group of Slayers in Italy, and later doing more Watcher work in Scotland. Xander is also working with the Watchers, although he doesn't want to be called one, and Robin Wood, also American, is a Watcher as well.
It's possible that as history's largest and most populous empire, Britain was the logical location for the Council to base itself - secure and stable, global in reach and tended by a culture that also developed institutions like the British Museum, Oxford University and the Royal Society. My theory is, the Shadow Men 'evolved' from their African origins and entered ancient civilization via Egypt, transferring to Greek control after the conquest of Macedon, then to Rome after Julius Caesar conquers Egypt. After the fall of Rome, the Council tracked the movement of power and wealth to Western Europe, perhaps via Germanic/Saxon or Norman migration, and eventually settled in England. In the season seven episode Never Leave Me, a Watcher named Phillip mentions - among other locations - Council bases in Munich and Rome. It's possible these may be former headquarters from earlier stages in the Council's history.