The charity has helped countless people over the 20 years it has been running, and Grisenthwaite was recently celebrated for the work he has done so far.
He was lured to the shop in London to do some “internal filming” – and had no idea what was actually planned.
“I was happily doing the spiel and next minute I saw Alexander Armstrong pop up in the corner of my eye,” he said.
“He said: ‘I’ve got a flat tyre’.
“I knew nothing about it, my team and everybody around, it was a gotcha moment.”
Armstrong was there with BBC One’s The One Show, as part of their “One Big Thank You” series.
He played Grisenthwaite a series of videos of children who have benefited from the charity as well as hospital teams and people working at the charity shops.
Then the biggest surprise – a video message from Sir Chris Hoy, a moment he said was “off the scale”.
“He’s going through his own journey with cancer and I was just so touched,” he said.
“It was amazing exposure for the charity as well.”