The term "hoover" being used interchangeably with "vacuum" in British English is a classic example of a genericized trademark. A genericized trademark occurs when a brand name becomes so commonly used that it replaces the generic name for a particular product or service.
In this case, "Hoover" originally referred to the Hoover Company, an American vacuum cleaner manufacturer founded in 1908. The company's vacuum cleaners were so popular and widely recognized that, over time, the term "hoover" began to be used colloquially to refer to any vacuum cleaner, regardless of the brand. This process is known as genericization.
The Hoover brand was well-known and successful in the UK, so the term "hoover" became ingrained in British English to describe vacuum cleaners. It's similar to how people in some regions use "Xerox" to mean photocopying or "Kleenex" to mean facial tissue.
Even though the Hoover Company is still in existence and continues to produce vacuum cleaners, the term "hoover" has become a generic word for vacuum cleaners in the UK, much like "vacuum" or "vacuum cleaner" is used in other parts of the world.
It's important to note that the use of "hoover" in this context is specific to British English, and in other English-speaking regions, people may use the word "vacuum" or other regional terms to refer to the same cleaning device.