I have wondered about that as well. The D5 even looks too small for a family of five now. I suppose they can manage a FFRR as it is at least a bit wider. One of the Jaguar cars might work as well, otherwise back to Audi I guess.
In the jpg, but not in the video is one of the D4's security is still using. What will replace them is now a real question.
At least in the D4 you could get equipment and people in the rear portion. Maybe MI5 or whoever will have to transition to Suburbans, but I suppose the new G Wagon is the logical replacement.
Wait, there is hope! Rather than being the Australian Ford Ranger, the new Defender could be a rebadged LR4. Miracles could happen.