Could have been. The Royal Flight aircraft are going to be painted all white by the end of the year, as a security measure. So maybe the aircraft you saw was half way through the job!!! By next week, perhaps the tail will be white too and the Union Jack gone.
Pity we have to take such anti terrorism measures in this day and age. The Queen will look less "regal" stepping off a plain white plane!
It will resemble something like the sub charters that the likes of Flightline operates to Gerona on behalf of Britannia when one of their 757's is sick.
AVIACO