This may be related, it was certainly unusual. 10:30 this morning near High Wycombe there was a rear-mounted twin jet at (very roughly) 20,000 feet heading SE. It made a 180 degree left-hand turn and headed off NW. Looking at the vapour trail, it appeared to have come in heading SW and made a 90 degree turn onto the SE heading described above.
I couldn't imagine why an airliner (round here it would have to be an SAS MD8x or a BMI Embraer) would do this, or a business jet. But now I've read this thread, I recall reading that the RAF use private jets (HS125?) as 'targets' for training purposes. Anyone got a better (more sensible) explanation?