Mario, root beer is a dark, carbonated beverage very much like cream soda. Not sure if the UK has anything like root beer but some brand names here are Dad's, Graf's, A&W, Frostie, and smaller production, higher quality brews. There is no alcohol in root beer.
Bottles that do not have a seam going to the top have what are called "applied tops". Sometimes the top seam is very faint or even ground so it appears there is an applied top when in fact there isn't. I would be surprised that the bottle with that wire bail closure has an applied top. That closure fitting would need to be uniform to seal and I think that is a machine made bottle all the way to the top. I do not think it was for root beer or soda. This bottle would have had a paper label which is long gone. I suspect this was a food product flask for oil or something. The top closure looks familiar and may be more recent than suspected.
Could the bottom markings be U G B? [United Glass Bottle Manufacturers, Inc. (large conglomerate of many glass factories in the United Kingdom). Mark dates from 1913 to about 1968.] The "8" on the bottom looks like a "B" but the other letters are too faint in the pics.
The top sealing mechanism is called Kork-N-Seal. "The Kork-N-Seal cap was in use from at least 1911 to at least the 1960s, though could still be in use today (IGCo. 1911; Jones & Sullivan 1989). Based on empirical observations, the popular period for this closure - and the most likely date range for a bottle with it - was from the mid 1910s to the 1940s. Bottles utilizing this closure are, in the experience of the website author, always machine-made, though with an origin no later than 1911, mouth-blown examples are possible." http://bottleinfo.historicbottles.com/closures.htm#Lightning "This closure/finish type has been observed on medicine and pharmaceutical bottles, narrow to moderately wide mouth food bottles (like sauces), household bottles (polish, cleaners), liquor and wine bottles, and to a limited degree carbonated beverages - soda and beer."
