I was not able to check this on my computer at the time and found no mentioning about this at the aviation enthusiasts’ forums. However, when the US Navy Boeing E6A Mercury was flying around the UK for several days in a row at the end of April 2011, it was on FlightRadar24. I saw it myself.

A good calm weather and clear skies above England ensured that the unusual contrails were spotted far and wide across the country, reported by local media, discussed at various forums etc. Pictures and videos were taken, and the planes were identified. The racetrack contrail was left by RAF Sentinel R1, ZJ692, whereas the circular contrail came from NATO E-3A Sentry, LX-N90454.

Places were the racetrack contrail was sighted include Worcester in the West (about 100 miles from my place), Farnborough in the South and Crewe, Cheshire, in the North (these two places are 140 miles apart). The reports of AWACS contrail sightings came from several places in a surrounding area of least 80 miles across.

While browsing through numerous reports, I noted that the observers generally underestimated the scale of these contrails and assumed that the aircraft circled the sky much closer to their places than it actually was. It was educational for me too, as before I did not fully realize that some contrails that I can see from my place are farther south than London and farther west than Birmingham.

Here is another attempt to share my observation made yesterday, January 16th, 2012. The pictures can be seen above in my posts on 5:01 pm and 5:45 pm.

I was lucky to observe a long racetrack contrail and an AWACS circular orbit contrail at the same time (the first and second pictures). It was a fair contrail day. The contrails were long but did not persist for long, apart of some more remote areas of the sky. This is why you do not find many of them on the satellite image of the area, taken at 10:45 UTC, some 20 minutes before the beginning of my observations:
http://lance-modis.eosdis.nasa.gov/imagery/subsets/?subset=AERONET_Chilbolton.2012016.terra.250m

Nevertheless, the racetrack and circular orbit contrails are visible. I highlighted them on a Google Earth screenshot together with my viewpoint (third image). Both contrails cast the shadows on the ground, allowing the estimation of their altitudes, 12 km and 9 km, respectively. Unusually, there appeared little wind up there, less than 10 mph, resulting in the AWACS contrail coils being quite close to each other. This suggests that the contrails did not move much by the time when I noted them. Indeed, knowing the exact position and FOV of my camera, I readily fitted on Google Earth the contrail models based on the satellite image into my photos (the fourth and fifth images).

The racetrack contrail was from some 50 miles away from me at the closest point to more than 100 miles at the furthest point. Being about 100 miles long it just fitted into the widest angle (~70°) of my camera. The circular orbit contrail was closer, between 20 and 50 miles. Zooming on the plane at the closest point (on the two consecutive turns some 12 minutes apart) I got the images (sixth and seventh ones), showing that the plane does look like Boeing E-3 Sentry (AWACS).

The last, eighth image is a photo of the same or similar plane passing right over my head two days earlier.