If a human offered an 80% discount, the humans employer isn't magically forced to honor it.

Firstly, it depends whether the offer satisfies the criteria of "an offer" in a legal sense. Unless purchases for 8,000 GBP are generally negotiated in a chat window on a website then this kinda seems unlikely. Anyhow, let's assume it is an actual "offer".

If the offeror renegs on the contract, they may be liable to a claim from the buyer for the costs incurred by they've incurred as a result of the bogus purchase.

For example, if the buyer dispatched a truck to the vendors warehouse and on arrival they were told the sale wouldn't go through, then the vendor might be liable for the cost of the delivery truck for a few hours.

In reality, no one is going to bother making a legal claim for that small amount of money.