As a UKer, a better question would be "Why are burger buns so hard to find?". Seems like no matter which supermarket you go in, there's an 80% chance they won't have anything suitable for putting a burger in.
Thousand Island Dressing is a pretty close approximation of Big Mac Sauce. I used to work at Pizza Hut and every now and then I'd make a "Big Mac Pizza". It was a pan crust pizza that used thousand island dressing as sauce, cooked with cheddar and beef topping. Then when it came out, it was topped with lettuce, onion, and cut up pickle, similar to a taco pizza. Very good, I've made it myself a few times since leaving the pizza business.
It's really easy to make yourself. I go with mayo, ketchup, mustard, worchestershire, oregano, cayenne, paprika and salt. And if my wife isn't having any, I'll add some finely chopped pickle. It's good stuff.