My knowledge is very poor of that time period. The best I know is 'Fall of Civilizations: Roman Britain' on youtube, something like that would be great or anything would do.
You're referring to the period known as Sub-Roman Britain? Just clarifying so it can help others narrow down the resources they know of. Usually the period is said to begin with evacuation of Roman troops to Gaul by Constantine III in 407 and to end with the Battle of Deorham in 577, though I've seen c400 - c600 and c410-630, for example.
This 3.5-hour documentary from History Time on Youtube goes into this period in detail, but it does bleed into the beginning of the Anglo-Saxon period towards the last 45 min: After Rome - The War For Britain
Here's a significantly shorter lecture about Sub-Roman Britain entitled...er...Sub-Roman Britain ... from the Anglo-Saxon England podcast.
The "Fall" of Rome & The "Arrival" of the Saxons - Early Medieval History
Post Roman Britain: Irish and Germanic Invasions - lecture delivered as part of the Bilkent University course HIST-416 Medieval British History, 2009-10 (alternate link - Bilkent course HIST-416 Medieval British History, lecture 4: Sub-roman Britain: migrations and settlements - starts around 16:00; you'll need to scroll down to the lecture title on the playlist, there is no direct link to the video.)
Re-Thinking the Emergence of the English - lecture delivered on behalf of the Chester Archaeological Society by Cambridge Professor Emerita of Archaeology, Susan Oosthuizen (article about her book from Current Archeology)
It's a tricky period to pin down for documentaries and videos: the creators all seem to want to blend periods together somewhat seamlessly as there are few reliable sources for this specific period and the dates seem variable. I hope this is what you're looking for. I'll root around for more if you'd like.
Hi there anyone interested in recommending things to OP! While you might have a title to share, this is still a thread on /r/AskHistorians, and we still want the replies here to be to an /r/AskHistorians standard - presumably, OP would have asked at /r/history or /r/askreddit if they wanted a non-specialist opinion. So give us some indication why the thing you're recommending is valuable, trustworthy, or applicable! Posts that provide no context for why you're recommending a particular podcast/book/novel/documentary/etc, and which aren't backed up by a historian-level knowledge on the accuracy and stance of the piece, will be removed.