As a store manager for a relatively large retailer, i’d say thief is a little strong however you have been dishonest and should return the item.
The fact it’s £600 worth is key as most retailers wont sit back and let you take it without a fight, especially as they’ll have proof you haven’t paid. I assume it was signed for when delivered?
If we wrongly delivered £600 worth of stock in a similar scenario, i’d be straight on your back to get the funds or the item back. Were it £30 i’d probably just suck it up. The fact you ordered it in the first place suggests you can afford it.
I think you know the right thing to do.