What is the current hand luggage requirement for Heathrow, for a return trip to the USA?

My husband and I are leaving the US for a week in London this Friday. From Atlanta to Heathrow we can have 1 personal item and 1 carry-on bag on Delta Airlines. I’ve read conflicting info on what is permissible as hand baggage from Heathrow to Atlanta on Delta, that only one piece is permitted per person. Is this true? Any help would be greatly appreciated!