Virgin destroyed my credit score

Hello. Last year at the end of September, I called virgin media to cancel my contract. I have been subscribed to virgin media for 5 years and I have never had a late payment. Someone from virgin media told me that I had to notify in advance to cancel it and I had to pay an installment once more. I paid and after 2 months they inform me that I have to pay again. I called and they said that they can't look into my account until I pay. I paid again and they saw that the request was submitted but they did not close my account. That woman promised me that she would hang up on me and that I would receive an email. In the spring of this year, I received from the BPO that I have to pay at virgin. And I called and again my account was not closed. They also told me that they would close it the other day and I received that I have to pay. I called and was told that my account was closed in April 2023, 6 months after I submitted the request to close the contract. In the meantime, I checked my credit score because I would also like to buy a house with my family. I have 2 children and one has autism and I struggle hard to raise money and apply for a mortgage. And virgin media shattered this dream for me. I was up-to-date and their incompetence ruined my credit score. What did I do wrong if I have the right to choose and they took revenge by ruining my credit score and my dream of buying a house. I have all the records with everyone I talked to and they confirmed that the application was submitted last year at the end of September.