Because people have a habit of using an internet forum to answer question(s) they could answer themselves, if they bothered to engage their brain and use an ounce of effort on their part.
It tells you on the label how to complain about quality of the product. It's literally right on the packet / label of every single food product in the UK.
As such, it suggests the OP did not create the thread to ask "how do I complain about this" but to get attention of fellow forum members.
Also a forum is for discussions, not spoon-feeding lazy individuals the answer to an obvious question which everyone should already know the answer to, all they needed to do was read the damn label on the bottle.