Well, you just blew your own theory, didn't you?
First, you said "Welsh or "British" law wasn't incorporated into English law."
Then, when you took the trouble to look it up, you found that you were wrong.
You also found out that the argument made upstream, that English law is "really" Danelaw, was also false.
Good job!
I did, because it didn't. English courts needed to know Welsh laws to make rulings based on them when dealing with Welsh subjects. Their laws weren't made universal, IE: English. Where Welsh laws were in conflict with English law, English law replaced Welsh law. Use of Welsh law in Wales & therefore English courts was discontinued in the 16th century.
Then, when you took the trouble to look it up, you found that you were wrong
Gotta be using some very twisted logic to come to that conclusion, but then again, I seem to be dealing with someone who believes it appropriate to bring credentials to the table instead of a case.
You also found out that the argument made upstream, that English law is "really" Danelaw, was also false.
I saw the claim as hyperbole, mostly a counter to the common claim about Scandinavians being only barbarians, contributing nothing of value to Europe. The Jutes, Angles & Saxons bumped up against each other on the mainland for generations & claim common ancestors. I don't think it is knowable whether or not an understanding about laws came before or after the tribes split. Could be they were created because the tribes split, where blood feuds could become a threat to the existence of all.