I have never been in a Subway retail establishment. In fact the list of food outlets I have never entered is extensive, they include McDonald's, Greggs, KFC, Domino's, Pizza Hut, Papa John's, Starbuck's, Costa Coffee, Caffe Nero, Nando's, Zizzi, Pret A Manger, Wetherspoon's to name but a few.
I did once go into a Pizza Express, it was quite an unpleasant experience that won't be repeated.