The player, currently on loan at Dewbsury Rams from Featherstone Rovers, will join up with his new club in 2015.
“Ben is a utility back who can play centre and wing and he comes from a good programme having been brought up by Warrington Wolves,” said London coach Joey Grima.
Hellewell believes that the switch to a full-time environment can only help his development as a rugby player.
“Switching to full-time training with the club will help my rugby career massively and after speaking to people about the coaching staff, they couldn’t speak highly enough of them,” he said.
“The main reason I decided to sign for the club was the enthusiasm of Joey Grima to transform the club into a winning culture. All the players and staff I’ve spoken to are friendly and welcoming which I found inspirational.
“I’m looking forward to playing with every single player at the club. London have made some great signings this year in players I’ve watched when I was learning the game.
“I’ve also watched a few of the youngsters coming through the youth development system at the club that have made an impression in the Super League and there’s some more coming through to look out for in the future.”