I am from the United States of America, so I do not have a British accent. I would recommend watching T.V. shows or movies starring British actors/actresses so that you can copy their way of speaking. Listen to a key phrase that interests you and copy it over and over until you think it sounds correct. From there, you could have someone listen to your pronunciation and critique it. I hope this helps and I hope you have a great day.
Best regards,
Howard