My husband, who was living with his family in London in the 1960s, reports that they had barley water there, too. It was bottled and carbonated like a soda, and typically lemon-flavored. A quick follow-up with Wikipedia uncovered the following:
Barley water, usually flavoured with lemon or other fruit, is a popular British soft drink. It can be made by boiling washed pearl barley, straining, then pouring the hot water over the rind and/or pulp of the fruit, and adding fruit juice and sugar to taste. The rind may also be boiled with the barley.
Drinking boiled barley in water, strained or not, is an ancient practice. Barley water has been used as a first baby food, before feeding with barley mush. It is also used to treat cystitis. In Mexico drinks called aguas frescas are made by street vendors using similar methods. Roasted barley tea is a popular traditional Asian equivalent.
One of our local supermarkets carries the British-style lemon-soda-like barley water. Although that's not going to taste much like anything the Vikings or Anglo-Saxons might have drunk, it is the end of a long tradition that may well have involved the Vikings (who certainly used barley). I'm also curious as to how British barley water differs in taste from typical American lemon sodas.