Time immemorial is 1189 CE
Do you know how many documents survive from 1189 CE? F#*k all, that’s how many.
We don’t have the original case law because, quite simply, we don’t have the original case law. In any event, “time immemorial” is simply a legal fiction stating the date to which you had to be able to trace land ownership before you had indefeasibility of title. Common law goes back way beyond that and even way beyond the Norman conquest in 1066.
At a certain point, it doesn’t matter. Things are done that way because things are done that way. For example, Pythagoras’ Rule (the square of the hypotenuse of a right triangle is equal to the sum of the squares of the other two sides) was not discovered by Pythagoras. It was known to both the Babylonians and Egyptians 1,000 years before Pythagoras was born yet it will likely be Pythagoras’ Rule for as long as the human race endures.