The 78-year-old comedian, was diagnosed with Parkinson's disease in 2013 and retired from live performances five years later, but he has continued to record programmes and make TV appearances.
The Scottish star, also known as The Big Yin appeared on the Graham Norton Show via video-link from Florida, where he lives, to talk about his new autobiography, Windswept & Interesting.
He told Norton: "I have lost the ability to write, and it breaks my heart as I used to love writing letters to people.
"My writing went down the Swanee and is totally illegible, so I had to find a way to record everything, but then the recorder didn't understand my accent so it kept collapsing and my family would have to sort it - it was a club effort!"
Explaining the title of the book, he said: "The rules of being 'windswept and interesting' are doing as you please and not taking lessons from anyone."
Sir Billy gave an update about living with Parkinson's and said he has "good days and bad days".
"It's creeping up on me and it never let's go. I walk like a drunk man and have to have help. So, life is different, but it is good," he said.
Other guests on the show include Doctor Who star Jodie Whittaker, Olympic diver Tom Daley, actress Dame Eileen Atkins, and comedian and writer Sir Lenny Henry, with a musical performance from Coldplay.