The laws are a sop to Unions
Despite dwindling religious fervor in the UK, Trade Unions prefer restrictions on larger corporations - ensuring they can only open up to six hours on Sundays. This is only to the detriment of consumers, who are often disadvantaged by having to pay more for a product, or travel further to get it, because trading laws shut down stores near them. After uproar over the Uber vs. Unions saga, this would be an important step for the government to show they aren't in the grip of overly powerful Unions and actually work for individuals.
Power should be less centralised
It shouldn't be up to a central committee or even the government to set laws for regions of the UK that Ministers don't know about or understand. Instead, companies should be able to decide for themselves, based on commitments to both customers and staff, whether to open or not. Some companies pay staff extra to work on Sundays, and this works for both the customer and staff member.
The UK is getting more multicultural by the day
Old religious laws that keep Sundays 'special' seem quite pointless now that the UK is an eclectic mix of religions and cultures, and indeed fewer and fewer young people are growing up dedicated to Christianity. Whilst it is important to respect those who practice Christianity, many believe the choice should be up to them - and non-Christians / atheists shouldn't be restricted to the same old laws that don't have anything to do with their own religions.
It would boost the economy
Bar the freedom arguments, relaxing/abolishing Sunday Trading Laws would help give a boost to the economy. More tax revenue collected, more income and profits for companies, and a bigger pay packet for workers who are happy to work Sundays. There'd also be a likelihood of the creation of more jobs - with more working hours to fill.
Perhaps a long shot, but bear with. Opening up stores and therefore shopping on Sundays would help ease the congestion on Mondays - some would take that day off work, or indeed, do their weekly shop on a different day than usual. We've all fallen victim to ridiculous Monday supermarket queues - at the hands of archaic Sunday Trading Laws.