Hmmm....a zoom in indicates that it has some sort of structure and tapers to the left. Part of the object appears to be in shadow. I therefore don't believe that it is a weather balloon as I don't think it was spherical. Also those things usually drift at very high altitudes and I don't think they are released around here. (I may be wrong about that).
If it had been a helicopter it probably would have generated a lot of noise. As Jeane guesses, it may well be a small plane or glider, but it really is very shiny. My guess is that it could be one of those odd shaped silver children's balloons strongly reflecting sunlight, but I'm far from certain.
You can see that it is not circular from the attached pictures where I've played about with the contrast.
If I may add, it's a very nice photo notwithstanding our Unidentified Dover Object!