Erm .... after you buy, you get a "reasonable time" to inspect goods to ensure they conform to contract. How long that is will vary, according to circumstances and the nature of the goods.
But the issue is, after delivery, whether you "accept" the goods or not? This is accepting the goods, in contractual terms, NOT about accepting or signing for delivery, which is entirely a different matter.
There's several ways you can "accept" goods, such as by telling the seller you do, or by doing things (like selling them, or customising/altering them) that aren't consistent with not having rejected them, or simply by not rejecting them in a "reasonable period". And that last bit is where the rejecting goods that don't conform to contract in a time period comes in.
And IF goods don't conform to contract and you reject on that basis, you are entitled to a FULL refund, because the supplier is in breach of contract for supplying goods that didn't conform to contract.
After that "reasonable period" expires, or if you've legally accepted the goods in other ways, then we end up falling back on the SoGA in terms of satisfactory quality, fitness for purpose, as described, etc
and that is where the repair, replacement or (possibly) partial refund bit comes in.
But for goods arriving dead, and rejected pretty much immediately because they're dead, and therefore don't conform to contract, you're in full refund territory .... though that period will be fairly brief. Very likely, a few days, and other than in exceptional circumstances, a few weeks (typically, with a month at most) .... but it could be a LOT longer if circumstances dictate that, however long, it's still "reasonable". That's why it's the court that decides ho long reasonable is.