In answer to your question, absolutely, this is how I discovered the character. I didn't even realise that there were Batman comics until by chance.
Same here! In the U.S., the show could be found in weekday afternoon syndication in the mid-'70s. I knew what a comic book was--I think there were some Caspers and Richie Rich's around the house--but I've always distinctly remembered the day that I was browsing through the comic books at the local supermarket and had the epiphany of a lifetime--"Mommy! It's Batman--in a comic book!
I own a copy today, though not my original. Cover date August 1975--would have come out around May. I would have been 5, finishing nursery school (an optional pre-kidnergarten at my elementary school). Yes, I could read it, thanks to a Grandma who taught me very early...though I didn't understand all the words.
My first exposure to Supes was the George Reeves appearance on I Love Lucy
--none of my local channels were playing Adventures of Superman
reruns at the time--followed by Super Friends
. SF was also a strong influence, as I remember some of my subsequent early purchases being issues of Justice League of America
(who I knew from a guest appearance on SF back when it was the big five with Wendy, Marvin, & Wonder Dog).