That enables you to lose the £440k instantly and with no recourse instead.
- lose key to wallet
- lose wallet due to inadequate backups
- lose contents of "brainwallet" due to weak password
- lose wallet/contents due to malware targeting bitcoin
- lose wallet/contents due to bug in client software (recommendations change frequently)
- send to merchant, defrauded in the old-fashioned way
- keep on exchange for trading, exchange collapses
- erroneously swap transaction value and transaction fee, or other fat-finger errors
- erroneously send to wrong or non-existent target address
Sending to the wrong address can be fixed with smart contracts that associate addresses with easier identifiers. Swapping value/fee can be prevented with a simple safety check in the client software, to make sure the fee is reasonable.
not possible with bitcoin
On the other hand, I by mistake sent bitcoin to a wrong address and couldn't believe that someone who owned that address sent me all bitcoins back week or so later. There are good souls out there :)