Currently in his ninth senior season with the Blues, Davies has made a total of 114 appearances for the region.
Phil Davies, Director of Rugby at the Blues, said: "Bradley has been a fantastic professional during his time with the Blues. He is a real team player, a very popular member of the squad and has been a wonderful servant to our region.
"We are disappointed to see Bradley leave. He has been brought up through this region and he has done a great job for us over the years."
"We wish him and his family all the very best down in London, I am sure he will be a success playing under Dai Young."
Davies added: "I have been with the Blues since I was 17- years-old and I have a huge amount to thank everyone there for, but I am also hugely excited about playing in a new league with a new club."