Although Pytheas of Massilia voyaged to Britain and probably circumnavigated the island in the 4th century BC, his account of his travels was not well received by his contemporaries. My review of Barry Cunliffe's account of Pytheas' voyage can be read here.
So what did Romans know of the geography of Britain? Julius Caesar visited the island twice, and left us with this account of its geography. Strabo included this information about Britain in his Geography, which Sarah Pothecary dates to 17 - 23 AD.
Tacitus includes some incidental information about the geography of the island in his biography of Agricola, who had been governor of Britain. During his governorship Britain was circumnavigated. One of Plutarch's philosophical dialogues, has a character named Demetrius who had come from Britain where he had been sent on a voyage of exploration by the emperor's order (see Chapter XVIII).
Ogilvie and Richmond in their commentary on the Agricola suggest that the information in Ptolemy's Geography about northern Britain derives from Agricola's time in Britain. For what Roman maps of Britain would have looked like, see romanmap.com and have fun with the maps and place names.