Longstaff made his international debut in 1994, going on to win 101 caps for Britain and late last year helping them reach the final Olympic qualifier.
He called time on his international career prior to the tournament in Riga, making the decision in January, but has just completed an EPL league and cup double with Guildford.
Longstaff has since joined Britain's backroom staff for the Division One Group A World Championships in Budapest, which begin on Sunday.
Britain are currently on a training camp in Manchester ahead of the six-team tournament which will see them face hosts Hungary, Kazakhstan, Italy, Japan and South Korean.
Only the winner will earn promotion to the elite division and Longstaff said: "I am very proud and honoured to be asked to join the GB staff by Tony Hand.
"I loved playing for the national side and now to be back with GB again is fantastic. We have a great group of guys here and I am looking forward to being involved."