My first reaction was to take the easy way out and ask Martyn Cornell. Unfortunately, he didn't know either. There was nothing for it but to start looking myself. The obvious place being the British Newspaper Archives.
Just searching for "Burton Ale" wasn't going to be very productive. I'd just find hundreds of adverts for Bass and Allsopp. Reckoning brewery price lists were my best bet, I went for a dual search: "Burton Ale" and "Mild Ale". As I was only looking for London brewery price lists, I also limited the search to just London publications.
I kicked off in the 1860s. Plenty of references to Burton Ale, but none of them brewed in London. I'd just about given up on the 1870s, when I came across this:
|Tottenham and Edmonton Weekly Herald - Saturday 24 May 1879, page 8.|
This is what will appear in "Free!", my book after next, on the topic:
Stock Ales Had been brewed in London for a considerable length of time. Even the big Porter breweries had been bashing them out since the 1830s. Initially called XXK, XXXK and XXXXK, by the latter part of the century they were mostly known as KK, KKK and KKKK withing the brewery.
There was also a change in what they were sold as down the pub. With the name Burton gradually coming into common usage. By the time WW II rolled around Burton was the term universally used in London for strong, dark beers.
Why the term was used is a bit of a mystery. Most likely it was on account of strong Stock Ales for which Burton-on-Trent was famous. And had been long before IPA was ever brewed in the town
The when I can do a bit better on. Trawling through the British Newspaper Archives, the first mention of the name in connection with a London-brewed Strong Ale I could find was from 1879. In an advertisement from the Collier Brothers brewery of Walthamstow, which lists a “Fine Strong Burton Ale” for 60 shillings a barrel. That price indicates a very powerful beer, probably 1080º, at least.
A pair of price lists from the same brewery give a clue to when the adoption of the term Burton began. In 1876 Santer and Collingwood, of the Albion Brewery on the Caledonian Road in London advertised a “Strong Ale” for 4 shillings and 3 pence a dozen imperial pint bottles. Four years later, the same beer was advertised as “Strong Burton Ale”. Which places the term’s origin at the end of the 1870s.