I saw one on the Greenwich Peninsula next to the O2 and another one while leaving Paddington towards Slough.
Other posters have correctly answered that these are gasometers. But it's worth noting that they are no longer used at all; they were built when the UK mainly used town gas, which needed to be stored, but since the 1970s we have switched entirely to natural gas, which is piped in directly. Gasometers are now obsolete, and many of them have been demolished. Some are the subject of campaigns to save them, as examples of the UK's industrial heritage.
The technology is called a Floating Head Tank. The idea is that the tank is sealed around the bottom by two walls with water in between. As gas is pumped in, the tank rises. The weight of the tank keeps a constant gas pressure. Typically, the tanks would gradually fill overnight meaning a large capacity was available locally for daytime use yet with a small pump capacity.
They're gas holders or gasometers - they're used in several cities around the world for storing gas. See Wikipedia for more information.
Tom Scott made an episode of "Things You Might Not Know" about these gasometers.
Gasometers, which store natural or town gas.