under UK law, if an item breaks and they can't repair or replace like for like, they have to give you a refund "minus fair usage"
the current market value of the item doesn't come in to it, but regardless of the warranty period, if a reasonable person would say that an item should last for 5 years and it breaks beyond repair after 1 year, you should get four fifths of what you originally paid back
with graphics cards, manufacturers tend to either replace with the same model, or failing that an item which is equal to or slightly above the same specs now, so you wouldn't get a 980, but a 970 would not be an unreasonable expectation, but ONLY IF they can't give you a 780