Davies progressed well through the Blues Age Grade system making his competitive senior Blues debut against Edinburgh in January 2006.
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."
Bradley Davies added, "I have been with the Blues since I was seventeen 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."