How to make a formal complaint?

I would like to make a formal written complaint. I have contacted Apple by telephone and was told to complete the form on apple.com/feedback, which does not provide sufficient space to make my complaint. I have visited the store and was told by the manager to visit the same website. I was also advised and provided with the stores contact details. However, the manager did state that the complaint would remain internal within the store.

I do not find that this is acceptable and I would like to make a full written formal complaint, without having to send a letter to the US.