Got a Olympus camera for my daughter for crimbo from Argos, when I got it one of the things I was informed was that it was not part of the 28 day return scheme, it could only be returned if it was defective.
As I said the shop can not waive your statutory rights just because they decide to sell something cheaper, it is not your concern that they may have sold below their cost, one of the following may be true
1) The product functions identically as the product not on the offer, you still have a statutory right to 28 days
2) It is not fit for purpose, e.g. substandard as the OP or fails
3) Subsequent to purchase something came to light that was not evident when offered for sale
Frankly this all part of the cost running a retail business and as such is an operational cost. These costs can be offset against profits ergo against taxation.
Companies like Currys have agreements with their suppliers to take back all returns in some cases up to two years, you may not believe this but these large retailers often return around 30% of the electronic goods they purchase the final cost is borne by the Manufacturer and ultimately the consumer in higher pricing
So yes we have strong consumer protection but likely adds 10% to the cost for everyone
And one last little gem, your contract is with the shop not the manufacturer. Some manufacturers do offer a direct warranty service, you may chose to use this service, however the shop can't force you to do so, or they would be in breach of the 'Sales of Goods Act'.
As an example A friend of mine bought a PC from a large retail store, it failed on or about 21 days, I happened to visit a couple of days later and he told me that the store had given him the manufactures helpline to sort it out. He had phoned and they had arranged to pick up the PC but he was not to pack in the original packing because it gets thrown away at the repair centre :| , a courier was due the next day to collect. My reaction "are they taking the P", after a polite conversation with the store and following my advice, he reminded the store that they had to meet the requirements of SOGA, subsequentally He picked up a brand new PC in store that evening.
In most cases when you have issue the shop staff are not trained in regard to the SOGA, or the Manager/Owner thinks they can ignore it, after all its time consuming and they can't take money whilst dealing with your complaint
On the contrary much as we malign the large DIY shed they have a 45 day refund policy, I have never had any issues on returns, it's dealt with politely and efficiently they see it as good business, and that is the point, it should be