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Cleistocactus (or Borzicactus) samaipatanus, AKA Samaipaticereus boliviensis, depending on how far back in time you read. It's in a large plastic pot on top of my mailbox, which is 16" / 40cm square.

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I've had this since about 1995, when a friend in the Henry Shaw Cactus Society gave me a tiny cutting. It's one of the easiest of all cacti to propagate, grow and flower. In St Louis I grew and flowered it in a 3.5" / 9cm square pot.

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It tolerates near-full sun here, almost all day long, if it's watered regularly. It also does really well in a hot, humid summer climate. It doesn't mind being moist all summer. That is what it gets in habitat. Still, it shouldn't stand in water. It flowers multiple times in summer once stems get about 2 feet long. More fertilizer means more flowers. There are multiple clones in cultivation. None is particularly upright, though many people try to stake them that way. It looks best hanging in a basket, or from a pot on a pedestal.

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It tolerates a fair amount of overnight frost without damage. It needs the next day to be well above freezing, however. I don't think this would grow in a garden very far north.

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Any stem cutting will root easily during warm weather, in bright shade. Water, then let get almost dry. If you need a lot of plants in a hurry, stems can be cut into slices of the same length as the diameter of the stem, and pressed sideways, half-way into the potting mix.

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I have five other Cleistocactus species, all of which are very upright growers. They also have wonderful flowers. One is red-orange, one red, and two are pink with bright green tips. One has fluorescent lime-green flowers that open at night.

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Mourning doves build their nests in a lot of my flowerpots this time of year. I wasn't able to get close enough to focus properly.

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After this flowering I will prune the pieces dangling down towards the opening of my mailbox. If anybody wants some send me a message.

I know it is grown in England, by members of the British Cactus and Succulent Society. I think it is readily available from specialty cactus nurseries in the EU as well.