I never really liked Weetabix very much, despite it being a childhood breakfast staple (I was even a member of their mid-eighties WEETABIX CLUB. How I wish I'd kept that membership kit!). But I did love their run of SF and comics licensing tie-ups in the early 1980s (I was too young for the seventies DOCTOR WHO card inserts, which have subsequently become legendary.
I remember the DC Comics masks that you could cut out from the back of the box (I had The Flash. I had no idea who he was... but he had - by a mile - the coolest mask). I'm not sure when they ran that promotion but - I assume - it was in the late seventies and pegged to the release of SUPERMAN THE MOVIE (although the license was for DC's comic book superheroes - Superman, Batman, Wonder Woman et al - rather than their screen offspring).
Another favourite was Weetabix's STAR TREK: THE MOTION PICTURE tie-up. That was a series of artwork character cards featuring the regular cast and - in a nod to the Star Wars generation - the Klingons and other barely seen on-screen aliens.
This particular advert (from March 1981) promoted the FLASH GORDON CARDS (18 to collect) inserted into packs. It's not made clear here but - as I recall - the larger the pack: the more cards that were inserted inside. That was certainly true for the STAR TREK promotion and - I assume - the same was true here.