Joe Alwyn's isn't looking very Merry this Christmas ... riding a Lime e-bike on Boxing Day in Britain -- with a downcast expression on his face.
The actor was spotted flying around London on the rental bike Thursday ... looking a bit worse for wear the day after Xmas.
He's bundled up in a black jacket and pale blue jeans -- hair flapping in the wind -- though he's not cracking a smile while rolling down the road.