LONDON—British supermarket group Asda is to extend its trial of a refill scheme that allows shoppers to replenish their own containers after sales of several products in an initial pilot store outsold packaged alternatives, it said on Wednesday. Asda, Britain’s third-biggest grocer by sales after market leader Tesco and Sainsbury’s, said it would expand its refill proposition to four more stores by the end of the year, having launched the concept last year at its store in Middleton, Leeds, northern England. Britons have become increasingly aware of the amount of plastic they use in recent years, with television documentaries such as David Attenborough’s “Blue Planet II” particularly highlighting the dangers of plastic pollution to marine life. In response supermarket chains and consumer goods groups are increasingly testing refills as part of a drive to cut single-use plastics. Earlier, this month Marks & Spencer extended its packaging-free refillable grocery concept “Fill …