United Airlines and British Airways are primary nonstop operators on the ORD to LHR route in December, offering multiple cabin classes, lounge access, and frequent flyer benefits. British Airways typically offers strong connectivity at LHR with BA Club and First options, while United can provide seamless connections within the U.S. network and Star Alliance benefits. American Airlines may offer seasonal or codeshare options; check alliance partners for mileage credit. Consider seat comfort, onboard services (meals, Wi-Fi), and winter reliability when booking. During December, view cancellation policies and flexibility features carefully and opt for refundable or changeable fares if travel plans could shift. CTA: Compare December fares and amenities across airlines to choose the best flight experience for your ORD to LHR trip.