All the comments so far seem to believe that the Sunday Trading Hours are to do with Church - they're really not. Not since 1994, anyway.
Before 1994 shops just couldn't open on Sundays, at all. That was probably because of the Church. When the 1994 Act came in, big shops had their hours restricted and little shops didn't. This was because the big supermarkets were already putting little shops out of business - they had the economy of scale and could undercut the small retailers.
It was felt then that the big supermarkets were well on their way to being an oligopoly (like a monopoly, but spread between a small number of players); and that if they were allowed to drive all their smaller competitors and completely control the market, it gave them too much power. They were already in a position to be 'price makers' and not 'price takers' - meaning that they told suppliers how much they would pay for their goods, rather than suppliers telling the supermarkets how much they would charge them for their goods. If you can force e.g. farmers to sell you their milk at below what it costs them to produce, when you've got 60% of the market - what can you do if you control 100% of the market? It also opens up the possibility of collusion, where all members of the oligopoly watch each other's prices and when one puts their prices up, so do the rest. Effectively, choosing not to compete any more, because they can all make more profit that way - together.
So that's why, in loosening Sunday trading hours, big retailers still had restrictions beyond the small retailers. Because if they didn't, they'd drive all the small retailers out of business, dominate the supply chain, control the market by operating an oligopoly, and be able to charge us whatever they damned well pleased, and we customers wouldn't be able to do a thing about it because there'd be no-one else to buy from.
So, no - these laws are not 'old-fashioned' and 'inconvenient' - they are what keeps the big retailers in check and competing on price with each other.