Undertaking Negotiations on Investment and Trade for Economic Dynamism Act or the UNITED Act This bill grants certain authorities to the President for entering into a comprehensive trade agreement between the United States and the United Kingdom (UK). Specifically, the bill directs the President to seek to initiate negotiations with the UK regarding tariff and nontariff barriers affecting any industry, product, or service sector. The bill authorizes the President to enter into a comprehensive trade agreement with the UK, with such authority expiring on March 1, 2025. Further, the President may proclaim a modification or continuance of any existing duty or continuance of existing excise or duty-free treatment to carry out an agreement, with certain limitations. The bill also requires the President to consult with and notify Congress regarding the intention of the President to enter into an agreement or make a proclamation.