This writing is from the start of the historical section of the text that includes the Wonders, the Historia Brittonum, and is taken from Manuscript A, widely considered to the oldest copy of the Historia. In full, the first line is "Brittannia insula a quod[am] bruto consule romano dicta" ("The Island of Britain is called after one Brutus, a Roman consul"). The text is from a facsimile given by Stevenson*. You can see the other facsimiles he gives on the Manuscripts page.