In short, no, it isn't illegal to not offer a replacement.
The non-short version is rather more complicated. You are, under certain circumstances, entitled to require them to do so, but there are plenty of reasons why it might not apply.
Consumers have a variety of rights, but exactly what they are very much depends on what has gone wrong (or rather, why it went wrong), and when.
Generally, for the first week or so, you can send stuff back under the Distance Selling Regs, but that doesn't apply here because of the time period. In that case, it's more or less down to the Sale of Goods Act (and various bits of legislation that amend the original act).
You have a "reasonable" time to inspect goods, and reject them if they are unsatisfactory. That would entitle you to a full refund, but "reasonable" is likely to be a short period, quite possibly a matter of days, and other than exceptional circumstances, a month or so at most. So that's passed by here too.
After that, you'll be entitled to repair/replacement, refund (probably partial) or compensation. But ONLY if the problem with the goods is something that existed at the time the goods were sold. That includes problems with durability, so it may be a fault that existed when they were sold but that wasn't obvious, such as a faulty component that only actually died after a period. However, there's a critical 6-month cut-off period.
If goods are returned after that initial "reasonable" period but within 6 months, then it is assumed that the fault existed at the time of sale, and those remedies are available
unless the retailer can prove it did not. If, for instance, the retailer can prove (perhaps via independent inspection) that the problem is due to user error, then no remedies would be available and the retailer isn't liable. If, for instance, goods weren't used in accordance with instructions, or if they'd be modified, that would apply. If a cooling fan had been replaced for an alternative model to standard and it failed, or wasn't fitted correctly, or if a board had been OC'd and over-voltaged, there could be problems.
But all that is within 6 months of sale.
After 6 months, the burden of proof as to whether the fault existed at the time of supply or not switches. In order to have those Sale of Goods Act remedies, the consumer needs to be able to prove that the fault DID exist at the time of supply. If you can't do that .....
And even if you can do that, you have the right to choose replacement/repair, but the retailer is entitled to decline if the cost of doing so would be disproportionate to other remedies .... or indeed if repair/replacement is impossible. There are some other twists and turns in who has to do what and under what circumstances, such as the nature of any cost difference in relation to the significance of the defect, but that's the essence of it.
In other words, you can insist on a replacement or repair
provided such is :-
a) possible
b) not disproportionately expensive.
And, once that initial "reasonable" period is passed, the retailer is perfectly entitled to reduce any reimbursement to reflect the usage that the consumer has had from the goods before they failed. For instance, one such "fair" reduction would be to say that if, for instance, an item was generally regarded as having a 3 year lifespan, and it failed after 18 months (due to a fault that existed and could be proven to exist when they were sold), the refund would include a deduction of 18/36ths.
Note, this is not a depreciation charge. It's charge to reflect usage and therefore value already received.
The above is all based on legal rights. In addition to that, the consumer would also be able to rely on any warranties the retailer offered, such as Scan's warranty, or a manufacturer warranty. Neither of those reduce your legal rights, but can add to them or provide an alternate route.
But ..... if you've got an item that's more than 6 months old, in order to be be able to stand on your legal rights and start requiring repair/replacement/refund, etc, you would need to be able to
prove, should the case go to court, that the fault existed at the time of sale. If it developed afterwards, due to user error etc, you aren't entitled to any of those remedies.