I'm particularly looking for things about the cultures of the various groups before the invasion as well as what the invasion was like for the Romans.
Tacitus wrote a biography on his father-in-law, Agricola, who was governor of Britain around about ~ 75-85 AD, which includes a fair amount of stuff about Britain back then. That might be worth a read if you fancy some original source sort of stuff.
Obviously, it could be considered a bit of a fluff piece seeing as he was writing about a relative, but still, pretty interesting stuff.
Simon Schama's "History of Britain, vol 1" dedicates a good chunk to pre-Roman/Roman occupation culture in the UK. I highly recommend it.
Neil Oliver's "A History of Ancient Britain" is good. The accompanying BBC series is also really good if you can get it, presented by Oliver himself.