This isn't really a history question, but an engineering one. The history question of where did they all go is ever so slightly too new (19 years!) to be an Ask Historians topic, as you will see.

The structures are gas holders and they still see some use today, although modern gas holders have a more modern appearance (looking something like a cross between a water tower and an oil storage tank). Their function is to store and balance gas (coal, natural) pressures in the gas fuel systems that feed cities. They were essentially giant bladders whose caps would piston up and down to store/feed gas into the city's gas piping network.

It was only in the 1990s that they became functionally obsolete in Britain, and began to be phased out. (The decision to start demolishing the inventory of old gas holders came in 1999, so technically too recent to be an Ask Historians subject, but whatevs.) Here is a BBC article on the demolitions, which has an historical overview to help understand their purpose.