1. An excess on an insurance policy is the amount a person has to pay before a claim is reimbused. Almost all vehicle insurance policies have an excess. It helps to lower the cost of the insurance policy. If your policy does in fact have an excess of £450 and you should check this for definite, it means you must pay the first £450 of any claim. As your claim is only for £320, then this means that you must pay the £320 yourself. So I regret to say that if there is an excess on your policy, then you will have to pay.
So the person who hit me (admitted liability) gets away with paying nothing
2. No, you can sue them for the £320 damage caused. So you should be making a claim on their insurance, not your own.