I suppose the Britons by Christopher Snyder in this sub's booklist is also fine for non-specialists. Anyway, if you wish to read accessible/ readable overview of the period in question, I'd recommend:

• Salway, Peter (ed.). The Roman Era: The British Isles: 55 B.C.-A.D. 410. Oxford: OUP, 2002. Short Oxford History of the British Isles 1: offers a balanced overview of the period, with a chapter specially dedicated to the 4th century Roman Britain. Alternatively, 2nd edition of his Very Short Introduction series (Roman Britain) is not so a bad choice either, I suppose.
• Charles-Edwards, Thomas (ed.). After Rome Oxford: OUP, 2003. Short Oxford History of the British Isles 2: roughly covers the 5th to the 8th centuries Britain, not only concentrating on the classical narrative between the new invaders and the 'Britons'.
• Fleming, Robin. Britain after Rome: The Fall and Rise 400 to 1070. Harmondsworth: Penguin, 2010 (2011): is probably the most readable academic history book on Anglo-Saxon England by the single author. It begins with the 1st chapter on Late Roman Britain, and the 2nd chapter deals with the 5th and 6th centuries, about 30 pages per a chapter respectively.
• Oosthuizen, Susan. The Emergence of the English. Kalamazoo, MI: ARC Humanities, 2019: provides the latest academic discussion on the fluid ethnicity ('Anglo-Saxon'/ 'Britons') in Post-Roman Britain in an accessible manner, though she seldom allude to historical Arthur and its historical background

Thanks!