Here's one that's bound to turn up in a pub quiz: which is London's oldest blue plaque? Well, let's hope your quiz master knows it's not a straightforward answer.
The blue plaque scheme was introduced by the The Society of Arts in 1866, with the tussle for the first plaque between such luminaries as Benjamin Franklin, David Garrick and Lord Nelson. In the end, Lord Byron came up trumps, his birthplace at 24 Holles Street, Cavendish Square, becoming home to London's, and the world's, first official blue plaque in 1867.
In 1889, however, some bright spark decided to demolish Byron's first home, along with the plaque (today Holles Street is rather an eyesore altogether). That means the oldest surviving blue plaque in London is that on 3 King Street in Westminster, marking the spot where Napoleon III lived in 1848. The plaque was also installed in 1867, but later on in the year than Byron's.
Michael Murray-Fennell of English Heritage (which took over the blue plaque scheme in 1986) tells us that Napoleon III's plaque is the only one to be unveiled while the person celebrated by it was still alive; the Emperor of the Second French Empire died in Chislehurst in 1873. You could argue that Napoleon's plaque is something of a sycophantic act from London's high society; he was really only here because of his exile from France.
Though Napoleon's blue plaque is round in shape and very much blue (see main image), it took some years for this to become the norm. William Wilberforce's 1906 plaque in Battersea is square with a highly decorative border, a plaque dedicated to art critic John Ruskin in 1925 is unpainted metal and nailed to what was a garden fence, while a plaque in Blackheath marking the former home of Philip, 4th Earl of Chesterfield is embedded into the masonry of the house.
While Byron's plaque was never officially replaced, there is now a green plaque on the John Lewis store that occupies the writer's erstwhile address. It bears the Byron quote: "Always laugh when you can. It is a cheap medicine".