Guy Halsall's Worlds of Arthur and Robin Fleming's Britain After Rome: The Fall and Rise 400 to 1070 seem to be well regarded on this sub after a quick check of the search bar.
Prior instances of someone recommending one, the other, or both:
edit: A bot has requested that I ping /u/MonsieurKerbs and /u/alriclofgar so that they can rightly get credit for their excellent replies in those threads.