The Broadway legend -- responsible for the numbers heard in "Cats," "The Phantom of the Opera" and others -- dished to WaPo about the Royal assignment he agreed to about a month ago ... namely, composing THE musical piece that'll be playing while KC gets crowned in May.

It's quite the task, and yet ... ALW sounds super chill about it. His interview here makes it seem like the gig is just another day in the office. He says, "I have written quite a lot of choral music. [M]y father was a very distinguished church organist and choirmaster and the choral tradition is not something that is new to me in a way, because it’s been there all my life." In other words, he's been there ... done that. Now, he's just doing it for the King.

As WaPo notes, Lloyd Webber's actually somewhat familiar with Charles -- having met and visited with him on a handful of occasions in the past. He tells this funny anecdote, kinda poking fun at the King's vanilla demeanor/interests ... and what HRH is passionate about.

ALW says, "The great thing about Charles is that he has a great love of really quite a lot of causes that are a little bit unfashionable. He cares very deeply about all sorts of quite interesting things."

The guy adds this ... "I mean, like, three or four years before lockdown, I got a call from him saying could I come around to meet him at Lancaster House quite urgently, because he’d had an idea. What it was that he was worried about was the fact that there wasn’t enough access for young people to go and learn how to play the church organ."

As for the actual music itself that he's putting together ... the deets are out. It's a three-minute tune set to the melody of Psalm 98, which will include the lyrics/refrain that reads, "O sing to the Lord a new song." Lloyd Webber's expected to put his own spin on it, though.

ALW's heading to London soon to test the audio at Westminster Abbey, where the ceremony will go down. Charles may be struggling with pop acts, but he's got the classics locked down.