Without having a crystal ball, it is difficult to predict what will happen in the future. Not just Brexit wise, but Covid-19 and its effect on most manufacturers’ component and their supply chains.
However, if you are happy with the current AP, and can persuade your dealer to re-order should the AP go down in Q1 2021, then I would say order now. At least you should get the vehicle you want with price certainty. If the AP went up in Q1 and you had waited, how hard would you kick yourself?
Also, the way vehicles appear and drop off the scheme with seemingly no ‘rhyme nor reason’ how disappointed would you be if your chosen car is not available on the scheme in Q1?