Regardless of getting them from a friend, they would still have a replacement value that needs to be insured against. So that's not going to save you anything. Perhaps you should explain to them, that you're not adding £700 wheels, but upgrading the standard wheels, which have nearly an equivalent value, for a different style. Tell them that you changed your mind since ordering the car, and now want the factory optional S-Spokes. Then ask what the insurance would have been if you had originally ordered them as factory equipment. That's what you should be charged.
But then, not everyone you talk to is going to empowered enough to really be helpful. You may have to be persistent.