The FH on Onslow's hat stands for Fulton Hogan Ltd., a New Zealand asphalt and road-building company. When Geoffrey Hughes was there promoting a show, he was given that hat by one of the company's lorry drivers.
During the series, Emmet and others are seen being frightened by Hyacinth's singing voice. In reality, Patricia Routledge is known for her powerful singing voice and has a long career in musical theatre.
In one episode Onslow and Daisy are in bed reading, conversing about DIY, when Onslow describes himself as being more intellectually than manually gifted. During this time he is glancing at a hardback book, the title of which is difficult to see on screen. The book is a newish copy of "Principles of Condensed Matter Physics."
The surname "Bucket" is repeatedly pronounced by Hyacinth as "Bouquet." All of the sisters' names are flowers (Hyacinth, Daisy, Violet, Rose) and so those names appear to be a tongue-in-cheek reference to a "bouquet of flowers." The maiden name of the ladies is "Walton."
Hyacinth's china pattern, described as "Royal Doulton with the hand-painted periwinkles," is actually a pattern called "Braganza" and was manufactured by The Colclough China Company. The Colclough China Company was founded in 1890 and was taken over by Royal Doulton in the early 1970s. (Hyacinth is only stretching the truth a bit.) The Braganza pattern was discontinued in 1992, and production, under the Colclough name, was discontinued in 1996. Hyacinth's cups and saucers were once valued at 20 pounds by Maxwell, Nixon, and Kray.