The price for the car, options, delivery charge is fixed when you place your order and pay the deposit, therefore you aren't affected by a list price increase for the product. The price you pay for vehicle excise duty however is based on the list price at the time of registration, not order, and therefore you are affected by a list price increase.
If your car price goes over the £ 40k threshold then your 2nd to 6th year road tax will have the £ 335 premium car tax added.