Parliament thought of that,jJack, and tried to deal with it, in providing that a borrowing is to be construed as intent to permanently deprive if it is for a period or in circumstances making it equivalent to an outright disposal, though the borrower did not mean to permanently deprive when he appropriated the property (s6 (1) ).
Curiously, when a person dishonestly takes a cheque, belonging to another, meaning to present it for payment it is not theft of the cheque form because, on presentation, it would be returned to the drawer by the bank : R v Preddy and Slade, R v Dhillon [1966] A.C. 815 H.L. ( House of Lords) That's the kind of decision which mystifies laymen !