The breathing tube in the film looks like a British-made J-shaped Typhoon T1 snorkel, which came with a "splash cap" which could be removed. At one time I owned an S-shaped Typhoon snorkel with a rubber cage at one end containing a ping-pong ball to shut off the tube when submerged. These snorkels were quite popular in their time, made of metal and sold by Typhoon (E T Skinner) until the mid-1960s, after which the company only offered plastic tubes. Typhoon was founded in the early 1950s by Oscar Gugen, who was also one of the founder members of the British Sub Aqua Club. Look him up in Wikipedia, he sounds like a larger than life character.
As for the nature of the metal used to manufacture the Typhoon snorkel, I have just done a little research in my modest library of books and catalogues. My 1966 Typhoon catalogue clearly states in the price list section:
Typhoon Alum. straight tube: 11s 0d.
Typhoon Alum. Ball cage: 16s 0d.
Typhoon Alum. Frontal: 10s 0d.
So there you are, the breathing tube was indeed made from aluminium in British English, aluminum in American English. There is a complicating factor, though, in so far as I recall these snorkels having a copper-coloured surface, while I associate aluminium with a silver hue. My copy of Collins and Chambers Ltd's mid-1960s catalogue lists the Typhoon T1 snorkel thus: "Typhoon single bend alloy snorkel with rubber mouthpiece and splash cap".
Perhaps the answer lies in another company's catalogue from the 1950s. Heinke, a British diving equipment company which merged with Siebe-Gorman in the early 1960s, manufactured a snorkel tube described thus: "This is a breathing tube in its simplest form and is of the type used by the majority of snork (sic) divers. The tube is of bent anodised aluminium and is fitted with a robust mouth piece". I expect the Typhoon snorkel was made from this material too, anodised aluminium alloy.
Barry J. Kimmins' book "Underwater Sport on a Small Income", published in London in 1956, has a page and a half on breathing tubes. He writes: "There are several types of breathing tube on the market, usually made of aluminium or plastic, and costing about fifteen shillings."
One last point. I brought the attention of British Sub Aqua Club forum members to the film and received more feedback than I was expecting. You can read the thread for yourself at
Diving into history: The film "British Sub Aqua at Thornwick Bay, Yorkshire (1957)" - BSAC Scuba Diving Forums
There seems to be some doubts in people's minds about the dating of the film as 1957, based on the images of motor vehicles seen in the video. 1962 may be a more likely dating. I'm afraid the gear worn by the divers doesn't resolve the dilemma either way. The snorkel tube was on sale until the mid-1960s, as were the fins worn by several of them: fixed open-heel Typhoon Surfmasters of the type popular with modern bodyboarding surfers.
Hope the above is of some interest.