This is the legal definition from the Theft Act 1968 (England and Wales):

Section 1 defines theft as: a person is guilty of theft if they dishonestly appropriate property belonging to another with the intention of permanently depriving the other of it.

Section 6 defines "intention of permanently depriving": a person is regarded as having the intention of permanently depriving even if they don't intend the other person to lose the thing itself, as long as their intention is to treat the thing as their own to dispose of regardless of the other's rights. This includes borrowing or lending it for a period and in circumstances making it equivalent to an outright taking or disposal.

So if you have evidence of the above then the police SHOULD NOT cast it off as a non crime, o civil or "they can't be bothered"