The subject of the first commercial broadcast on British television, at 8.12 pm on 22nd September 1955.
Gibbs SR, of course, was "tingling fresh toothpaste that does your gums good too. It's tingling fresh. It's fresh as ice. It's Gibbs SR toothpaste", as plummy BBC presenter Alex Mackintosh intoned in perfect Received Pronunciation tones. The advert was broadcast on the nascent ITV channel, which the BBC felt so threatened by that they coincidentally killed off a main character in The Archers the same night.
The advert was chosen by a process of drawing lots - there were 23 adverts in contention to be the first seen in Britain, and Gibbs SR saw off competition from the likes of Guinness, Brown and Polson custard, Batchelor's Peas and Brillo Pads.
The next day, Bernard Levin wrote in the Manchester Guardian - "I feel neither depraved nor uplifted by what I have seen... certainly the advertising has been innocuous. I have already forgotten the name of the toothpaste."
Sources:
A Short History of British Television Advertising - http://www.nmpft.org.uk/insight/info/5.3.55.pdf
TV Advertisements of the 60s, 70s & 80s - http://www.nostalgiacentral.com/tv/adverts.htm