I am sorry to hear about this unfortunate turn of events - I never book a client on a cruise without at least two FULL days at the point of departure prior to boarding the cruise for this very reason when the client chooses to book the air separately from the cruise line. When possible, I have the cruise line book the air and the line's own travel insurance policy which will fly the client to the next port to meet the ship. In the case of a missed trans-Atlantic crossing without a close-by next port, the insurance will fly the client home and refund the cost of the cruise plus expenses.

Did the travel agent discuss these options with you? Did you take their advice? If so, this is all on them, not United. If the agent tried to push you to book the air through Cunard with the insurance package and you refused, then this is unfortunately on you.