I'm looking to learn more about this period which I know incredibly little about. I'm mostly looking for something that's more of an overview than anything too in depth, but I'm very interested in this semi-mythical period of history. Really, my only current understanding is that the Romans left, Angles, Saxons and Jutes came, kingdoms were established and they fought with each other a lot. The religious aspect of it is also of interest to me, having so many of them in the small landmass of Britain is quite curious!
If you're looking for the origins of that story, then you should start with Bede's Ecclesiastical History, written in northern England ca. 731. It's our earliest source for most of that semi-mythical history, and once you get a feel for who Bede is and how he writes, I think he's pretty fun, too. He wrote in a Latin style that was easy and colorful, and although the Ecclesiastical History is widely available in translation, not all translators have been equal to the task.
If you start with Bede, make sure you find a translation that works for you. If you're doing this for research, the old edition edited by Colgrave and Mynors (1969) is still the best. And if you end up wanting to know more about Bede the brilliant polymath and stringent disciplinarian, check out the lengthy chapter on Bede (100+ pp.) in Walter Goffart's Narrators of Barbarian History (1988).
If you're looking for a more recent take on early British history, I'd recommend Guy Halsall's Worlds of Arthur (2013) and Robin Fleming's Britain after Rome (2010). Both are written with a general audience in mind. Halsall will walk you through why we shouldn't trust Bede, then he'll offer you his alternative take. Fleming scraps Bede almost entirely, preferring to start from the massive archaeological evidence and then referring to texts as they pertain. She avoids some of the historian-on-historian conflict that Halsall excels at, but her close scrutiny of the archaeological evidence may occasionally go deeper than you'd prefer. I recently discussed both in a bit more detail here:
Halsall has also written an excellent survey of the Migration Period: Barbarian Migrations and the Roman West (2007). In my opinion, it's the best currently available. However, to be fair, there are plenty of historians who disagree with Halsall's approach and stick to the old story about a deluge of barbarians suddenly washing up across the Rhine. Some of these options include Walter Goffart's Barbarian Tides (2009), and anything by Peter Heather on the Goths. Most of these and other Migration Period histories come with a more Mediterranean focus, e.g. Chris Wickham's Inheritance of Rome (2010) and Bryan Ward-Perkins's The Fall of Rome (2006).
Returning to Britain, however, a few final recommendations that might suit your interest in religions. Be forewarned that these are scholarly studies of particular topics. I cannot vouch that they're the type of overview you're asking for. That said, they're all excellent texts, but make sure you take a long look at them before you buy, or don't risk it and get them from your library (always a good option!).
I hope that gets you started! For a period called the "Dark Ages," it's got a lot to offer and (hopefully) keep you reading. Good luck!