I just returned from LHR with my daughter and when we got to Customs I was asked if I had a Travel Letter for my child. She asked my daughter some questions and seemed satisfied. I found this pretty odd as I was RETURNING home to my wife. At LHR we weren't asked for this letter, go figure. This is the first time I've been asked for this when returning home and the Agent was shocked (and said so) that I had not been asked before.

Is this something that is being pushed by the authorities and can it just be a written letter from my wife?

(I do understand the need when LEAVING my home country but not when RETURNING.)