I’ve heard Daniel speak many times, including before he was in Teen Wolf. He sounds the same as Kol.
https://www.youtube.com/watch?v=GG9GNi12x7Y
I couldn’t find a clip from the originals to compare it to but seriously, he has a London accent irl and not when he plays Kol. He’s putting it on as part of the character portrayal.
I’m American, so I’m struggling to put the correct name to the accent he uses in the Originals, but it’s definitely not his normal accent. It resembles what’s often described as a cockney accent in books.
http://www.cwtv.com/shows/the-originals I don’t know if this link is region locked or not but you can try it.