I'm really surprised that a town the size of Horsham doesn't have any fast food restaurants at all, save for one lonely KFC in the Bishopric. There is the odd fish & chip shop and Kebab joint buno chains to speak of..
It did boast both a McDonalds and a Burger King, but they had both...
The best thing about McDonalds are their breakfasts, particularly the pancakes with sausage & maple syrup. I try not to indulge too often though.
As for the coffee you are right, it is good. I can remember the days when they used to give you free refills though!