HP is a brown sauce from the UK, and commonly just referred to as "brown sauce." It's hard to explain, but it's a great alternative to tomato sauce (ketchup.) You can find it at most British import shops, or even on Amazon. It's one of those fun deals where we pay $5-6 for a bottle that costs about a pound over there (roughly $2.)

I blame the people at Jake's in Plymouth, England for my fond love of brown sauce. They made the best sandwich with ham and chips (french fries) covered in cheese and brown sauce, then baked in the oven. Geez, I'm salivating right now.