Under English law a contract which is vitiated for unilateral mistake can only be void, not voidable. If it could be voidable then if you sold it third party rights could be taken into account in deciding the remedy. However, even if only voidable rescission would be the most likely remedy on the original contract anyway which would place you in no better position than if it had been voided in the first place! Either way, you can't keep it. But trust us when we say - the contract can definitely be voided here.
They won't rescue a bad bargain, you are right, but they will rescue where unconscionable conduct takes advantage of a mistake which they knew about. With a price this low, so called "Nelsonian Knowledge" (closing your eyes to the obvious) can be used to imply actual knowledge. Trust me, this contract can easily be voided. Voiding puts it as if it was never formed - therefore you retrun the laptop, they return the money... what's the problem with that?
The whole point of thie contract being void ab initio at common law is that, in fact, the laptop never was his - he took advantage of a mistake which he knew, and courts will not sanction this.
If you don't believe me, as I said - go to court with these "law firms".