HP Sauce is a brown sauce popular in the United Kingdom. British prop comedian and magician Tommy Cooper (1921-1984) was credited in 2001 for this joke:
     
“I bought some HP sauce the other day. It’s costing me 6p a month for the next 2 years.”
   
The joke was explained on the newsgroup uk.culture.language.english on July 26, 2001:
     
“HP Sauce is a common brand of brown sauce with a picture of of big ben on teh label - hence HP (Houses of Parliament) IIRC. HP is also the abbreviation for hire purchase - I don’t know what it’s
called in the US - a system of paying for goods in installments (often monthly over a period of 2 or 3 years).”
   
     
Wikipedia: HP Sauce
HP Sauce is a brown sauce originally produced by HP Foods in the United Kingdom, now produced by the H. J. Heinz Company in the Netherlands. It was named after London’s Houses of Parliament. Since its first appearance on British dinner tables in the late 19th century, HP Sauce has become an icon of British culture. It was the best-selling brand of brown sauce in the UK in 2005, with 73.8% of the retail market.
 
HP Sauce has a tomato base, blended with malt vinegar and spirit vinegar, sugars (molasses, glucose-fructose syrup, sugar), dates, cornflour, rye flour, salt, spices and tamarind. It is used as a condiment with hot and cold savoury food, and as an ingredient in soups and stews.
 
Wikipedia: Hire purchase
A hire purchase (HP), also known as an installment plan, is an arrangement whereby a customer agrees to a contract to acquire an asset by paying an initial installment (e.g., 40% of the total) and repays the balance of the price of the asset plus interest over a period of time. Other analogous practices are described as closed-end leasing or rent to own.
   
Wikipedia: Tommy Cooper
Thomas Frederick Cooper (19 March 1921 – 15 April 1984) was a British prop comedian and magician. As an entertainer, his appearance was large and lumbering at 6 feet 4 inches (1.93 m), and he habitually wore a red fez when performing.
         
Google Groups: uk.rec.motorcycles
Friday Humour : remember Tommy Cooper
Champ
Apr 27, 2001, 10:45:07 AM
(...)
I bought some HP sauce the other day. It’s costing me 6p a month for the next 2 years.
 
Google Groups: uk.culture.language.english
HP Sauce Joke?
JBG,
Jul 26, 2001, 9:46:01 AM
Can someone help a befuddled Merkin understand this joke?
 
> I bought some HP sauce the other day. It’s costing me 6p
> a month for the next 2 years.
 
Regards,
John Grosh
 
Einde O’Callaghan
Jul 26, 2001, 10:18:50 AM
HP Sauce is a common brand of brown sauce with a picture of of big ben
on teh label - hence HP (Houses of Parliament) IIRC.
 
HP is also the abbreviation for hire purchase - I don’t know what it’s
called in the US - a system of paying for goods in installments (often
monthly over a period of 2 or 3 years).
 
Regards, Einde O’Callaghan
   
21 December 2001, The Belfast News Letter; (Belfast, Northern Ireland), pg. 8:
TODAY, in memory of the only man in the world who could make people laugh by simply standing there, and also in an attempt to immunise you against the Christmas cracker jokes you will be reading in only a few days from now, we bring you the Tommy Cooper festive joke collection.
(...)
I bought some HP sauce the other day. It’s costing me 6p a month for the next two years.
     
Google Books
The Mammoth Book of Comic Quotes
By Geoff Tibballs  
London, UK: Constable & Robinson Ltd
2004
Pg. ?:
I bought some HP sauce the other day. It’s costing me 6p a month for the next two years.
TOMMY COOPER
 
Google Books
The Mammoth Book of Great British Humour
By Michael Powell
London, UK: Constable & Robinson Ltd
2010
Pg. ?:
I bought some HP sauce the other day. It’s costing me six pence a month for the next two years.
Tommy Cooper
 
Twitter
The Dad Joke Man
@DadJokeMan
I bought some HP Sauce yesterday.
It’s costing me 5p a month for the next 3 years…
2:46 AM · Oct 27, 2020·Twitter for iPhone