I suggest that Augustus was very busy consolidating power and that the British Kings were "chips in the bank", which could perhaps be cashed in one day. Certainly the way in which British monarchs defined their legitimacy by reference to Roman standards of polity and ethics (never mind conspicuous consumption) tells us that Rome was the arbiter of "corectness" even if remote and non-interfering.Tacitus and Strabo certainly look a little more even handed in the quoted passages, the difficulty is of course that GJCs account is like an account of a British Victorian punitive expedition dealing with recalcitrant , cunning natives. If you admire GJC for his combination of wit,style,bravery and brass neck (as I do ) then I think you can "re-translate" in your own head, to a degree ,allowing of a more undoctored/spun mental vision of the Gallic Wars.
Ive said before , remember the initial "invasion" (expedition) had the prestige (and technical difficulty) of the first moon landing in its own day. To even land and wave a gladius about was an act of prestigious bravado that reflected on the Roman world as a whole.So, if you were part showman , part usurper what more daring task need be done? Conquest was perhaps not really on his mind at all-merely a "demonstrandum".