Yes, it's a very common phenomenon. London=England to many Americans. They've just never heard of any other British cities or towns.
I don't know I guess I had the benefit of my parents and half my family at least knowing of Sheffield when I mentioned where DH was from. They were like "Wow, Sheffield Steel..." and wanted me to get them knife sets as if I'm made of money!

I'd like to get a set of Sheffield knives before anyone in my family, but I'm not holding my breath on that nor is it a priority purchase. But they do like their sets of coasters and plates that they now have that were given as wedding gifts.
Birmingham all you have to say is that's where Ozzy Osbourne's from.
Sheffield - mention knives (steel), Sean Bean or the movie the Full Monty.
Liverpool - the Beatles.
York - mention that the castle was featured in the movie Braveheart.
