Why would this encouragement to shop at a British supermarket be in an American film? Sure, Tesco used to operate in the United States for a while but under a different name. Another question: were the sub-titles provided by the American production company or did the BBC outsource them to a British company?
As I have said, it is a mystery.
UPDATE: One person who has looked at this film thinks that "Always shop at Tesco" is muttered as a throwaway line by the woman on the left of the screen. I still don't believe it. Why would a character in an American film deliver a slogan for a supermarket chain that is unknown in the USA? They wouldn't, of course. I still believe that someone inserted the slogan in the sub-titles before transmission by the BBC. Was it mischief or unauthorised product placement?