But when Comic Relief called again, he couldn't resist.
Billy, who lost both his legs in a devastating racing car accident when he was 17, says: "This is the year that took me to the edge. This is the most extreme endurance challenge on the planet."
This documentary follows the 25-year-old's bid to become the youngest ever double leg amputee to complete an Ironman World Championships in Kona, Hawaii, for Red Nose Day. Billy also has a world record in his sights, hoping to be the fastest double amputee to complete the iconic one-day endurance test.
It's a 140.6-mile course - comprising a 2.4-mile open water swim, a 112-mile bike ride in fierce cross-winds, and a 26.2-mile marathon in the Hawaiian heat.
Cameras follo...