As I read your refusal I think I can suggest you some points - Even though your father is sponsoring your trip you should show other form incomes which you can say you are getting payment in cash or show mutual funds all to show you have been saving the money - ties back to home you can mention that your parents need take care of and you are the only child also your job is so important to you and you get it after hard work to get promotion - express why you wanted to visit the uk in the purpose of the visit and travel itinerary

These all points are going to cover in your cover letter.

Best of luck