Last i checked (as of a year ago) you could get A&W and/or Mug at Pan-asian (Japanese/Chinese) supermarkets. It's imported from Singapore.
Also, you could get them (as always) at Kosher supermarkets-- likely imported from the states.
Interestingly enough, if you visit the Five Guys UK website, they have in their FAQs - "Why doesn't Five Guys UK have root beer?". They blame it on an issue with their supplier. To which I counter-- you don't have a good supplier then.