Labels need to be diffrentiated from meanings of words. If you do not believe God exists, there is a word that describes you. That word is 'athiest'. Nothing to do with a label. If you play guitar, you are a 'guitarist'. Nothing to do with a label. And so on. Mind you, labels are actually quite important. How they are used is where we need to be careful, not whether they are used.
"Most people don't know what an atheist is, probably even dear old Johnny". How can any of us know what most people do or don't know ?
Hmmmm......I used to be an atheist. I decided back in 1977 at the age of 14 that there was no God. I was very definite about that for years. I knew what an atheist was then, at 14. It's kind of stretching the bounds of believability to conclude that a well read, much travelled, world weary, longsearching, heavilly debating guy like Lennon wouldn't know, two or three weeks before his 40th birthday, not only what an atheist was, but whether or not he was one.