Believe it or not there were two reasons stated for making Sunday Trading limited when it was originally allowed.
1) The House of Lords wanted to be sure people had time to go to church on Sundays! Remember, many clergymen, including Bishops sit in the House of Lords.
2) The government had to be mindful of small shops who had traded quite legally for years on Sundays and who might be ruined by the pulling power of the big stores.
At the time I was passionatley against Sunday Trading because my work at the time included a lot of contact with retail groups and because i was a regular church goer.