There is so much that cannot be said about your question because it lands us squarely in the undocumented realm of prehistory. Even during the Roman period, what we don't know about traditional British society is far more than what we know. So it would be easy to respond by saying that we have no way of knowing the sort of entertainment a travelling bard would provide. With that caveat stated, we can attempt to speculate based on what we do know.

There is a lot of pre-modern documentation of the British and Irish bardic tradition, often linked with musical performance, but at the very least with the telling of stories. The most thorough documentation of this tradition is with the Irish seanchaí (plural, seanchaithe), but there are counterparts in Scotland, in Wales (where they were known as the cyfarwydd), and in Cornwall, where they were called droll tellers. For example, writing in 1602, Richard Carew wrote of a practice in Cornwall reminiscent of an older age of Irish bards: ‘Sir Tristram [who] led a walking life with his harp to gentlemen’s houses’. The association of harps with the oldest documentations of travelling bards seems to point consistently to music as well as storytelling, but more particularly to the harp. There are other examples of this, and we can look to James Delargy (Séamus Ó Duilearga, 1899–1980) for his definitive work on storytelling in general - but drawing on Irish examples specifically.

What we have, when the dust settles, are a number of primary sources, which can be taken to suggest a common, core tradition that is very old. But as we use those examples to reach back to a given century, and then the one before and before, the amount of faith we can place on those primary sources lessens. To answer your question, we need to use pre-modern sources and a few earlier references to imagine a world that long-since vanished.

So while we can attempt to understand seventeenth-century Celtic, bardic performances, we can only speculate about pre-Roman British traditions. Again, with caveats heaped on caveats, I would say the following:

There is a long-standing tradition - with lots of international examples - of people who travel and tell stories, relate news, and often sing to their audiences, all the while gathering gossip, which they can take to their next destination. It is not hard to imagine early British bards doing just that. They told legends (stories meant to be believed) they related the news of the day, and when the sun set, they told folktales (long, fictional stories, the novels of the folk). Musical talent is distributed unevenly among humanity, so we can imagine that some of bards did a lot of singing, and that others preferred the spoken word. We can also imagine that our pre-Roman British community became very excited by the arrival of a bard. It meant that the mundane of day-to-day existence was about to be relieved. It meant that they would hear some things about the outside world, and it meant that they were going to be entertained - and they likely had the same glee that we share as we anticipate the release of a movie we have been "dying" to see. We can imagine all of these things because that's generally the way people received storytellers in well-documented times. In 1980, I had a folklore student from a remote rural Alaskan mining town, and she described the periodic arrival of "Dirty Dave," who enchanted everyone with his stories. With the connections that link Dirty Dave to the nineteenth century, to Carew and Sir Tristam in 1602, or to the seanchaí of Ireland, it's not too much of a stretch to imagine all of that.

Sources include James H. Delargy (Séamus Ó Duilearga), ‘The Gaelic Story-Teller with some notes on Gaelic Folk-Tales’ (The Sir John Rhŷs Lecture, presented 28 November 1945; published 1946); and see his ‘The Gaelic Story-Teller – No Living Counterpart in Western Christendom’, Ireland of the Welcomes, 1:1 (1952), pp. 2–4. Dundes reproduced his original article in International Folkloristics, pp. 153–76.

See also Peig: The Autobiography of Peig Sayers of the Great Blasket Island, translated by Bryan MacMahon (Syracuse, New York: Syracuse University Press, 1974; originally published in Gaelic in 1938) and her sequel, An Old Woman’s Reflections, translated by Séamus Ennis (Oxford: Oxford University Press, 1962); and Henry Glassie, Passing the Time in Ballymenone: Culture and History of an Ulster Community (Philadelphia: University of Pennsylvania Press, 1982). Gun Herranen provides an example of a Finnish-Swedish storyteller, Berndt Strömberg (1822–1910), discussing the creativity of the tradition bearer. Herranen’s article, ‘The Storyteller’s Repertoire’, appears in Reimund Kvideland and Henning K. Sehmsdorf, editors, Nordic Folklore: Recent Studies (Bloomington: University of Indiana Press, 1989), pp. 63–69. And there is also George Denis Zimmermann, The Irish Storyteller (Dublin: Four Courts Press, 2001).

I cover a lot of this in my forthcoming book - which has been delayed and was to be released several months ago, but is slow-walking its release from the printer: The Folklore of Cornwall.