Bert's right about the floor space issue, but it's also dependent on the specifics of the trade in question. Most businesses can only trade for 6 hours on a Sunday (things like grocers, record stores, bookshops, clothes stores) although some companies choose to only trade for 5.
For some reason that seems to pre-date the Sunday trading laws other businesses can trade for longer - furniture stores, garden centres, restaurants and video libraries. Quite why is beyond most people, but it's tied up with employment issues as much as trading laws.