The protected NCB simply means your NCB won't be reduced providing you don't exceed the specified number of claims over a period. If the OP makes a claim now, he will still have his original NCB when his insurance expires in August and he will still be entitled to a discount on his insurance renewal quote. Of course, if he makes a claim now, his renewal quote would go up in August (by ~£150 apparently) but that should be after the discount is applied. If the OP made a claim and didn't have protected NCB, the renewal quote would go up by more than £150. The NCB will be still be transferable to another insurance company but they may take previous claims into account when protecting it.
Back to the original question: Since the repair is £500 and the excess is £250 then the OP is only going to save himself £250 if he makes a claim now. However it appears the OP will be paying more than this in increased renewal quotes over the next few years. It's tempting to suggest that the OP should try and resolve the matter without making a claim but he will have to make sure that the third party agrees to this and that no one else has reported the accident. If a whiplash claim is made at a later date, the OP is stuffed.