Caesar tells us plainly that he knew virtually nothing about it before he went:

"...[Caesar] thought it would be of great use if he should pay a visit to the island, have a look at the type of people there, and learn about the locales, harbors, and approaches, all of which were virtually unknown among the Gauls." (BG 4.20) (magno sibi usui fore arbitrabatur, si modo insulam adiisset, genus hominum perspexisset, loca, portus, aditus cognovisset; quae omnia fere Gallis erant incognita.)

In the next sentence he concedes that Gallic merchants did seem to have some knowledge of the isle, but only its sea coast. This was in reality certainly true, and there is pretty solid evidence of seaborne trade between Britain and the Continent in this time period, not to mention some population exchange. When he brought these merchants before him, most of whom he had just defeated in sea battles in the previous season, they understandable had nothing to say to him, and were not exactly eager to tell him anything. The Morini and Veneti in particular were skilled seafarers and certainly paid regular visits to Britain in the decades before the arrival of the Romans.

As far as legends or tales about the isles: Caesar does not tell us any, though they surely existed and he surely heard them as he prepared to go there!