The refusal to give something back which has been borrowed is not automatically theft. In some cases it might have to be resolved using the Civil Courts and you may need to see a solicitor for advice.
B borrows C's power drill. B keeps promising to return it, B does not intend to keep it and it is just left in the garage, but B doesn't get round to giving it back. If this is pure forgetfulness, which goes on for a fairly long period (despite reminders) it would not be theft. However, eventually there comes a point where C has been deprived of their property for so long that B has no excuse for not returning it (perhaps a year or more) and the Criminal Courts could consider if B had stolen the drill.
If you have a problem with someone who has borrowed your property and has not given it back, keep a record of all the occasions you have asked for it and then ask a solicitor to send a formal letter asking for the return of the property. If the property is still not returned, there may sufficient reason to justify making a complaint of theft to the police.
Every case will be different and it may be, for example, that the borrower claims the property was a gift. In circumstances like that, the police may not be willing to take action and you could go to the Civil Courts for help to get your property back. In the case of a borrowed item up to a certain value, it may not be considered economically worthwhile to pursue.
Below is a brief summary of the offence of theft, it is not intended to be a comprehensive explanation.
"Theft occurs when someone dishonestly appropriates (takes possession of or makes use of exclusively for oneself/someone else without permission) some property that does not belong to him or her and treats it as his or her own with the intention of permanently depriving the rightful owner of the property."
◾ B is sat on a park bench with their mobile phone on the bench next to them, when C runs past and takes the phone from the bench and runs off with it.
◾ B takes an item from the shelf of a shop and leaves the shop without paying for it (shoplifting).