That so weird ! in Denmark by consumer law if the item you purchased cannot be replaced to one alike, then you can accept to get another similar specs/performance item but you are in no way obliged to accept it. If the reseller cannot exchangep to an identical item you are entitled by law to get the full refund of what you paid for the item at the time, and that is no matter how much it's worth today - it is always the date of purchase that counts.
So yes technically if I paid 4500Dkk for a GTX 780 1 year ago and I cannot get an identical one, then I can get my 4500Dkk back and buy another card e.g GTX 980. You can say that if you paid 4500Dkk for the GTX 780 and you are then offered a GTX 970 valued to 2800Dkk, then it's a 1700Dkk loss on your behalf, simply put the new card doesn't have the equal value of the original purchased item and that is what counts. Therefore you are not obliged to accept such a return offer, but one can agree to it if they wish so.
Also any replacement you get whether is an identical GTX 780 or a new GTX 970 then you're warranty period by the reseller automatically gets renewed with another 2years ! Meaning if I return a 1½year old 780 and get a new replacement then I have a new 2year warranty period.
If this isn't how it works in the UK, then I am actually happy that we pay a bit more for our hardware