Shops which closed on boxing day this year included M&S, Sainsburys, Lidl, Home Bargains, The Entertainer, Pets at Home. Many Next stores also closed.
Thats at least 50% of the shops at my local big retail park, so of course fewer people are going out shopping.
Also the fact that we weren't able to see family last Christmas, might mean that people are more likely to have plans this boxing day or to have travelled to see people and stayed over on Christmas night to boxing day.