A previous poster has shared a link, quite early on the thread, clearly showing the legality around this situation. It is NOTHING like leaving the lightbulbs behind! It is illegal to keep it. Plain and simple.
regarding the person who suggested this could be the same as discovering the house is worth more than you paid for it - no, no it isn’t. A legally binding contract for the sale of that house for that amount of money occurred. As long as consideration happened, it is irrelevant whether or not it was market value. Even if the OP only paid 1p for the house and they suddenly discovered it was worth a million pounds, no crime has been committed. It is for the seller to choose how much they accept for the sale. It is for them to research and check how much it is worth. There is no legal or moral duty for a buyer to tell someone that what they are selling is worth more. There are many instances where houses are sold below market value deliberately, usually for a quick sale.
As for those that say they’d keep it and ridiculing the OP for handing it over, I really do hope you realise what an awful person that makes you. And the person who kept the £4.6k knowing the rightful owner is alive and well, has made up a lie ready to tell the rightful owner, and then laughed about it - well there’s a special place in hell for people like you.
There are some truly awful people on this thread. And I suspect the ones who said they’d keep it are not the ones struggling to put food on the table, or are unsure if they can afford fuel this winter. Because people in that situation have far, far more empathy for someone losing their money and the devastating impact that can have.