hi! you may be interested in these posts
Why did the regnal numbers for the Kingdom of England reset after 1066 but not reset upon creation of the Kingdom of Great Britain? - /u/Algernon_Asimov and /u/Skittles_87 explain that regnal numbers were introduced in England by the Normans
Why was Edward Plantagenet dubbed "Edward I" when he was the 4th Edward to rule England? - /u/Valkine looks more closely at Edward I
Does anyone have any evidence of Edward I being styled "Edward since the Conquest the first"? I have a vague memory of having seen it, but I can't call to mind where.