When listing the Kings and Queens of England, such as in this rhyme that many British kids once learnt, we generally seem to start at 1066 with William the Conqueror, despite there having been kings of a united England for some time before this. This is true of the numbering of monarchs too, in that Edward I is "the first" despite there having been King Edwards before.

I know 1066 was a seismic shift in English history, but William himself thought of his conquest as claiming his right to inherit the crown from Edward the Confessor rather than making himself the first king of a new kingdom. How/when did the convention start of understanding English kings to "begin" with William I?