Question is all in the title.
The Liverpool Airport wasn’t named for John Lennon until 2001, more than 20 years after Lennon’s death.
That’s a very good point.
I’d like to amend the question to include Liverpool/Speke airport and any other names it had before 2001.
I don’t have the answer, but I would look at some of their later trips to Hamburg for the possibility. At first they went by boat but later they were flying there. But maybe out of Manchester, so I’m not sure.