I don't think it would be Weetabix because The Skinheads characters were the main campaign at that time walking about like Madness: "We're Weetabix -- OK".
I don't think it would be Frosties as Tony the Tiger was the main character: "But Tony you've got to help - We're one rider short!"
Fruit and Fibre was Nick Hancock singing, "Apples, hazelnuts, bananas, raisins, coconuts, sultanas - now breakfast's really going with a swing".
Bran Flakes: "They're tasty, tasty very very tasty".
Ready Brek: "Central Heating for kids"
Special K: "Can you pinch more than an inch"
Sugar Puffs: Usually Honey Monster and Henry McGee.
Shredded Wheat: Abraham Lincoln
https://youtu.be/3FlwaahcnGw?t=771
Could it be Kellogg's OJs?
https://www.youtube.com/watch?v=eBe5VBrwhpM
I don't know if this was ever broadcast in the UK. I don't remember it.