When your order is lockdown, you can't amend anything for any reason, that is a 100% certainty. When I placed my order I selected the magride, sport exhaust and TPMS, this was when the all inclusive sport pack wasn't available. For me to now get the newly released sport pack, no options have to be added or removed so does not affect the build at the factory. Instead, Audi UK have been able to consolidate each of those options and for the purpose of the invoice will now show as 'sport pack' and thus the discount can be applied. It's just a bit of admin involved and nothing physically changes to the configuration of the car.
I am lead to believe that if you only selected the magride & sport exhaust and not the TPMS as well (as in your case), Audi UK will not be able to apply a discount to your order even though you've spent £2000 on two of the options where all three can be had within the sport pack for £1700. I think the reason is obvious, they can't simply give you a £300 discount for the difference and can only change the paperwork so your invoice also shows the sports pack for £1700, but by their definition, yours would not be a sport pack as you will not have the TPMS and so probably for legal reasons they are unable to do this.
With you selecting an exclusive colour, this will explain why your order is already lockdown despite an anticipated build week early next year. I do think you will still get £300 discount directly from your dealer as this will be small change to them.