they sell them at takeaway places here in the UK but not like McDonalds etc and I have never seen them on another menu, but they are mentioned more in some US movies, TV shows etc. Are they pretty commom/popular there?
Yes, they are. You can't get them at say McDonald's or Burger King, but at your "mid level" restaurants in the States, they are quite popular on the appetizer menu.
We can't get them at McDonalds or Burger King, but at other places cheese fries and chili cheese fries are pretty common. A lot of people will go to other places just to get cheese fries.