I can see M & S point.
Someone put a photo up and the only real difference was Cuthbert had fewer smarties on top.
So at a kid's party it's brought out, parents see it and assume it's Colin. Maybe think it doesn't taste so nice and say "oh M&S is going downhill. Even their Colin Caterpillar cake isn't as nice. I'm not going to get the dc's cake from there this year."
With the publicity, at least they hope people will be aware, so won't do that.
And actually, Aldi, if you're not trying to piggyback on it, do what the others did and make it a bit different. There's imitation and there's trying to make it identical. There's a difference.
And no, I don't shop at either.