Isn't this the difference between a voucher benefit and an expense claim though?

Let's say you make employee of the month and receive an amazon voucher to be spent "treating yourself" but instead use it to buy a gift for your spouse.

Is this fraud? You made no false claim about how you would use the money. You did receive it with a specified use though.

What if your employer gives you a £500 a year raise and states it is to cover the additional costs of work from home. But you already have a nice chair so you but groceries. Is that fraud?

Seems borderline.