Adding to the confusion :
I get both sets of sizes in Norway. Sweden and Denmark are almost 100% metric - as the UK is "imperial" - but Norway gets caught in between.
I have learned the hard way that you cannot use 13x18cm film in 5x7" holders - except that some of the time you can. By a staggering coincidence, the 13x18cm film is pretty close to being 5x7 inches, while 5x7" film is considerably smaller!
I now have holders for both sizes - as well as 4x5" and 9x12cm.