Viewing from a migration watchpoint in southern Britain, I got three pictures of a lone thrush heading south last week, initially thinking it was a Blackbird. However, the pale underwing - and probably the wing- and tail-length - suggest otherwise.
Fieldfare is obviously likely, but I can't reconcile the pale underwing in all three pictures with no sign of a pale belly or vent, and no suggestion of any paler areas on head or rump sides.
A bit of searching suggests that Ring Ouzels can appear to have a pretty pale underwing, and a young bird may not show any pale crescent on the chest.
Searching online for images of Blackbird shows that it's very hard to find a picture of the underwing of a bird in flight - and I can't say I ever see them much above the treetops. But the underwing coverts should be darker than the flight feathers, according to BWP.
Obviously the quality of the pictures is the main challenge here, but does anyone have enough experience with overhead thrushes to make a call?