BBC Grandstand was a big sports show that was started in the 50's that lasted until 2007. During this time, the BBC's biggest competition ITV developed its counter called World of Sport in 1965 and this lasted until 1985 when World of Sport was cancelled. A 45 minute time-slot was reserved for a show called "Wrestling", which featured men such as Giant Haystacks, Mick McManus, and of course, Big Daddy, among many others. The Big Daddy character debuted in 1972, and quickly became the star of the entire World of Sport show, and in effect, wrestling in Britain because the territories were taking heavy losses during that period. Wrestling was temporarily cancelled with the cancellation of World of Sport, but found a slot until 1988, when it was cancelled for good.
Now, Wrestling got an astonishing amount of views for British standards, amassing 12 million viewers a week, but Grandstand still managed to draw a larger audience for the most part due to it being BBC's possession (keep in mind, pretty much everyone in Britain owned a TV at that point) To make matters worse, while the concept of the Big Daddy character was good for television, the same couldn't be said for buy rates, and promotion dwindled. Other performers dissatisfied with Big Daddy's dominance went overseas, such as Dynamite Kid, Davey Boy Smith and Finlay.
The closest thing Big Daddy had to major mainstream exposure other than television is a comic strip in the Buster, which was a fair amount below the big British comics at the time The Beano and The Dandy, and I believe was surpassed by 2000 AD pretty quickly (I can't believe I'm talking about Judge Dredd in the WZT). What I'm trying to say is that Buster was far from the biggest comic strip in Britain, even at the time.
In conclusion, Big Daddy was the biggest British wrestler produced primarily in Britain, but he still wasn't that much. He got great television views, but BBC Grandstand still outclassed them. He really wasn't much of a live event draw either during his time at the top. Gigantic for British standards, but again, British standards pale to the overall impact of The Undertaker, who I will say changed wrestling gimmicks forever.
Like I said, Big Daddy is not El Santo, and at the end of the day, The Undertaker impacted the wrestling world more than Big Daddy did.