HP Sauce or generically "Brown Sauce" is not like A1, kinda similar but miles apart, it's basically fermented fruit in vinegar with lots of molasses like sugar and herbs and spices, it works amazingly with thick cut sausages and eggs.
If you were really wanting to get some HP sauce, get a normal HP sauce, and a bottle of
Daddies, different parts of England swear by either.
And while you're there get yourself some british
Lea & Perrins and if you can get hold of some, my favorite is
mushroom ketchup, I put it in every pie I make, any mixed beef etc for a salty full flavor.
EDIT: Oh! And the best of all the british spreads / sauces,
Marmite.