If I look back at others who have shared the Queen's car to church - HM the Queen Mother, Princess Maragret, HRH Princess Alice, Dowager Duchess of Gloucester - even Sarah and Diana were there. I just think the Queen doesn't want to be alone and why waste and open sit. Prince Philip won't accept it if he can walk. I think the only reason Camilla wasn't in the car for Christmas was that Charles wanted to met their adoring public. But I doubt we will be seeing Sophie driving with the Queen much longer or even at all.
I'm afriad to say the tide has official changed. If is going to get very difficult to even get info about Edward and Sophie from now. We have entered the era of Charles and Camilla, William and Kate and Harry and Chelsy. They are now the royal family - everyone else is surplus goods.