I had three return tickets for myself, my wife, and my infant (4-months old) from London to Tehran in August 2017. On my return date, I arrived at the airport with my family at 5:30 while the flight was at 9:20 in the morning. I went to the airport with my family to come back to the UK via a British Airways flight. At the check-in, the airline representative took the travel documents from me, and issued the boarding card for me, taking my baggage. However, and when he wanted to allocate a seat to my wife and my infant, he said that tickets are not matching. He told me that he will contact the respective department to solve the issue. After being kept waited for few hours in the airport at the check-in desk by the airline's representative, I was told that I cannot take the flight since the ticket type for my infant is "Premium Economy", and it is "Economy" for my wife and myself. While I already had successfully purchased the tickets from British Airways through “Travel Up” agency, and reserved the seats. Ultimately and after being involuntarily denied checking in, I took a taxi and went back to the city. During the day (on 28th August), I called the airline’s customer service in the UK and the agency, asking for a new ticket. I was expecting both to help and support, since the mistake was made by the airline and the agency, and I was confident that the status of my tickets is “confirmed” by the airline. Although neither the airline nor the agency did not take any responsibility on the phone to issue me and my family new tickets. I was expecting the agency to be more supportive for me as of their customer. Whereas they just did not care, saying that this is not their problem. The agency and the airline were just passing me as of their customer to each other. Then I purchase relatively expensive tickets for which I went under debt. I express my absolute dissatisfaction from the airline and from the agency, who did not care about me at all as of a customer. This was supposed to be the most comfortable flight of my life with my infant, whereas it turned into the most uncomfortable one.