That sounds like control freakery in the extreme! The idea that you should be able to influence the wages of a small sector of your workers by intervening in the politics of another country?
The Tories tried do to this under UK-wide legislation and on that basis the SNP are doing what they were elected to do, which is to look after the interests of the people of Scotland, and as they are entitled to do as elected Westminster MPs.
I'd have thought the far bigger issue was the decision of the Labour party to oppose this. After all there is only one Labour MP from Scotland and I'd be very surprised if it really were were the case that the majority of Labour MP's in England and Wales had major issues with Sunday trading - so the political machinations of Labour MPs representing the areas that would have been affected has had a far greater impact on the voting numbers than the SNP had.
The reality is that the Tories mismanaged this: aside from using UK-wide legislation when they knew they had home-grown rebels on this, and never mind the risk of SNP opposition, did they really expect support from the SNP so soon before the Holyrood election when the Tories are so unpopular here - 'SNP help Tory party reduce wages for Scottish shopworkers' wouldn't make a good headline for the SNP.