Here is the first TV ad broadcast in the UK was on ITV on 21 September 1955, advertising Gibbs SR toothpaste:

Fun Facts:

        [According to Wikipedia] The first television advertisement was broadcast in the United States on July 1, 1941. The watchmaker Bulova paid $9 for a placement on New York station WNBT before a baseball game between the Brooklyn Dodgers and Philadelphia Phillies. The 10-second spot displayed a picture of a clock superimposed on a map of the United States, accompanied by the voice-over "America runs on Bulova time."

This may be a copy of that spot (The voice-over was probably supplied by the network, thus, this is silent) : 

​Fun Commercials: