There are three things one may put on your chips in the UK ("french fries" to you Americans and a pathetic imitation of a chip) - salt and vinegar, Heinz Tomato Ketchup or HP Sauce.
The recipe for this brown sauce was invented by a Nottingham grocer, who upon hearing it had been introduced to the restaurant of the Houses of Parliament, named it HP Sauce.
However, other apocryphal stories about the origin of its name exist (see the wikipedia entry) but it is to be noted that an illustration of the Houses of Parliament still appears on the bottle's label.
Alan Davidson's Encyclopedia of Food notes its derivation from earlier ketchups and Worcester Sauce.
Other versions of this brown source existed in the '50s-'70s including PK Sauce - hence my brother, Pete, was named Kevin Peter to avoid the inevitable piss-taking of P. K. ... but that was before the invention of KP Peanuts. With a surname such as Gascoigne, and forenames Angus Robin, the opportunities for name-calling were not lacking (penny in the meter ...).
HP Sauce was taken over by Heinz in 2005 and its UK factory closed down the following year with operations moved to the Netherlands.
(As an aside, I am also rather partial to tahini or soy sauce on my chips).