Caesar's Gallic War doesn't provide with much information on ancient Britain.
A good part of it covers the battles and skirmishes between Britons and Romans and their Gaul allies (It's trough Caesar's account we know that Brittons still used war chariots while these were not part of continental warfare probably since centuries); and only a short chapter is describing Britain as a whole.

There's no sign that the roughly accurate geographic description given by Caesar (Britain as a triangular island whom western angle is slightly oriented toward Spain, with Hibernia half its size on the west) was ignored by Greek or Roman scholars : Britons and Gauls were two closely related people (either culturally or genealogically), and Gaulish merchants most probably gave relevant information to people as Poseidonius, who inspired Caesar, Diodorus or Strabo.

Maybe the second part of Caesar's account, really briefly describing southern British society as similar to what existed in Gaul could be an interesting counterpoint to the description he makes of "inner Britons" as basically promiscuous cavemen which while possibly derived from a genuine situation, is quite on line with the trope of "outer Barbarians" as completely devoid of civilization and rules that was applied to other peoples in the remote corners of the world. (There's as well the necessity for Caesar to isolate Gauls as a particularly civilized people among Barbarians, and thus ripe for integration and conquest; and Britons or Germans).

Overall, it can be wondered how much Caesar actually discovered about Britain, except that people he contacted directly weren't nearly as caricatural savage than the trope he uses for remote Britons would paint them.