In the 80's the laws were a mess for Sunday trading, you could buy items from a "Private Shop" but Supermarkets weren't open so you couldn't buy Baby
food (this is the example everybody used to use).
There were certain items you could sell and those you couldn't, which for shops that sold both could cause chaos. Halfords could sell car and bike spares, but weren't actually allowed to sell you a bike.
It all got a bit much at the end of the 80's / early 90's as certain shops wanted to open, many wanted a day off to be fixed each week and no one wanted to work long hours every single day.
There were all kinds of campaigns, from Keep Sunday Special to Religious groups. In the end in all came down to a compromise, which then became law in 1994 (Sunday Trading Act), Big shops can open, but for a maximum of 6 hours. Little shops can open if they want to with no restrictions. (the big retailers of course do open slightly longer as the first half hour / hour is for "browsing" and they won't actually put anything through the tills during this time).
This has worked fairly well for almost 20 years, and apart from it being relaxed in London during the Olympics last year it's been a constant ever since, and pretty much everybody keeps to it.
It's just one of those quirks that we live with, but look at it this way, there are 168 hours in a week, the big ASDA stores are open for over 140 of them. If during that time you can't pause or record Big Brother / Britain's Got Talent / EastEnders / TOWIE to get what you need to live on, and the only free time you have is 3am on a Sunday night / Monday morning, then you need to slow down a bit.