The texts I've read have been primarily concerned with the second invasion under Claudius in 43AD, but what were the initial reasons for Caesar's invasion in the mid 1st century BC?
While waiting for newer answers, I'd like to direct you towards this comment by u/QVCatullus on the motivations of the invasion as well as why it doesn't necessarily make sense to label the invasion as 'failed'.
As QVCatullus has rightly noted, Caesar's given reason for invading Britain was the Britons' support of hostile Gallic tribes. But there are several other reasons he might have invaded that are far more compelling and interesting.
Glory-seeking and wealth would certainly have driven Caesar to invade—these were powerful motivators for Roman magistrates throughout the Republican period and arguably made the Romans more aggressive since each commander sought to achieve some sort of conquest during his short term of office (Drogula [2015] examines this dynamic pretty thoroughly). Caesar was no exception, but, if anything, he played an even more dangerous game, because he had been assigned the province of Transalpine Gaul under special circumstances. Caesar's consular provinces, Cisalpine Gaul and Illyricum, had been granted to him for five years by the lex Vatinia of 59 BC, but he had been given Transalpine Gaul in 59 for the sole purpose of dealing with the Helvetii, which he accomplished rather easily in 58 (which corresponds with Book 1 of the BG), meaning that by 57 there was no legal reason for him to be in Transalpine Gaul.
Because of these circumstances, Caesar's political enemies in the Senate were quite eager to revoke his assignment to Transalpine Gaul (Suet. Jul. 24.1; Cic. QFr. 36.3), which would have effectively ended Caesar's Gallic War much earlier than he wanted. As such, Caesar needed to prove that there were other threats in and around Gaul in order to justify his continued presence there—threats like the Belgae to the northeast, the Gauls of the western coast, the Germans to the east, and the Britons to the northwest. Several other scholars have argued that Caesar's brief journeys into Germany were mostly intended to show him doing something new (no Roman up to that point had ever crossed the Rhine and gone into the German wilds, which Caesar depicts as boundless and filled with weird beasts), and that this would have earned him glory and helped secure his position in Transalpine Gaul (see Krebs [2006] and Schadee [2008]).
His expeditions to Britain were probably no different. He spends a fair amount of time in Book 5 talking about Britain's climate, topography, and size, probably because he was the first Roman to go there (again, a source of glory) and he wanted to emphasize that fact. I agree with QVCatullus's argument that the invasions can't really be classified as failures, because Caesar never intended to conquer anything—just as with Germany, simply crossing the channel and roughing up some of the locals was enough to qualify as a notable achievement.