To enter the UK, your passport must be valid for the entire planned stay. Passport holders will be able to use electronic passport doors upon arrival in the UK. For certain nationalities, the passport or other travel document must be valid for at least six months after the proposed stay in the United States. The six-month requirement does not apply to UK passports.
If you visit the U.S. In the U.S., your passport must be valid for the proposed duration of your stay. You don't need any additional period of validity on your passport beyond this period. To enter the United Kingdom, all US, Canadian, Australian, New Zealand and South African citizens must have a valid passport for the duration of their stay.
A passport will allow you to stay in the country for up to 6 months. The immigration officer may also want to see proof of your intention to return to your point of origin (usually a round-trip ticket) and of visible means of support while you are in Great Britain (credit cards accepted). If you plan to fly from the United States or Canada to the United Kingdom and then to a country that requires a visa (India, for example), you must obtain that visa before arriving in Great Britain. The best way to find out if your passport is valid for travel to certain countries is to check the travel advice section of the Commonwealth Foreign Office (FCO) website.
If the travel document indicates your nationality, if any, and is valid for traveling to the United States and returning to the issuing country, you can submit it in support of a visa application. A passport is a travel document issued by your home country that allows you to travel internationally. This advice reflects the UK government's understanding of the current rules for people travelling with a full “British citizen” passport from the UK, for the most common types of travel. Visit TravelHealthPro (from the UK's National Tourism Health Center and Network) for general advice on COVID-19 for travelers.