This is just outside Clapham Common tube station.
A quick internet search reveals that, during the 19th century, clean safe free drinking water for all was very scare. The Metropolitan Free Drinking Fountain was set up in 1859 to provide free drinking water for London's general populace. The foundation set up drinking fountains and troughs like this all over London which immediately became not only very popular but vital for a healthy population. As the infrastructure increased and drinking water became available inside the home, these fountains fell into disuse.
As for me, I wish there were still horses and cattle in London to stop and take a long cool drink at troughs like these.