The veteran front rower has played for 10 other teams before joining London, as well as Great Britain, in a 20-year career.
London will wear a special jersey marking the testimonial in their Summer Bash game against Sheffield at Blackpool later this month.
“This is my 20th year as a rugby league player, and at the minute I am enjoying my rugby more than ever,” Thackray told the Broncos’ official website.
“Along the way I’ve met so many great people and made so many friends for life, which wouldn’t have been possible without rugby.
“I’d like to say thank you to all the fans at each club I’ve played for – you’ve always been great to me and taken me in as one of your own, and that means a lot to me.”