The shops act 1950 mentioned elsewhere was a mix of new laws and cleaning up of old laws including the Shops Act 1911, which placed restrictions on the number of hours that could be worked by shop staff, creating a 'half holiday'. You can read the Hansard debate here, but it's pretty dry. It was part of a raft of laws being passed around that time improving workers rights.
This half holiday was fixed as being a Sunday by the Shops Act 1950.
I'm sorry to say that your grandmother is either confused about why she was fined, or is spinning porkies;
Enforcement action would be taken against the shop, not the customer. http://www.legislation.gov.uk/ukpga/Geo6/14/28/section/14/enacted
In terms of legal principles, if you break a law, then the law is repealed, it's rare to get any kind of remedy, as you still broke the law as it stood at the time.
The queen has only pardoned the following people since 1961.
https://en.wikipedia.org/wiki/Conviction_of_Michael_Shields Accused of murder
https://en.wikipedia.org/wiki/Derek_Bentley_case Accused of murder
https://en.wikipedia.org/wiki/Timothy_Evans Accused of murder
https://en.wikipedia.org/wiki/Angelo_Fusco IRA terrorist
https://en.wikipedia.org/wiki/Paul_Magee IRA terrorist
https://en.wikipedia.org/wiki/Dolours_Price IRA terrorist
https://en.wikipedia.org/wiki/Peter_Goggins 'deserter' in WWI
The Shops Act 1950 - available on legislation.gov.uk - made primarily for reasons relating to worker conditions, as noted elsewhere in this thread - stated that (in England and Wales)
Every shop shall, save as otherwise provided by this Part of this Act, be closed for the serving of customers on Sunday
Some categories of trading as well as some specific items were exempt (see the Fifth Schedule - "Transactions for the purposes of which a shop may be open in England and Wales for the serving of customers on Sunday).
A newsagent for example, could open, but a shoe shop could not.
If the shop ran a mixed business - say, a newsagent which also sold shoes (a purely hypothetical example) - they could open on Sundays, but could only sell you your copy of the News of the World, not a nice pair of stilettos.
The provisions around food were particularly complex. In broad terms, fresh foods (e.g. fresh milk, fruit, vegetables) and fresh meals could be sold; tinned and bottled foods could not. Most confectionery was exempt.
This meant that a fruit and veg shop could open on Sundays, but they could only sell fresh apples, not tinned ones, even if they stocked both.
Presumably your grandmother was at a shop with a mixed trade, and persuaded the shopkeeper to sell her the prohibited tinned item.
