Good photo. No significant grain or blur on my screen.
I have had similar rejections when the light is flat and gloomy (at Manchester too!). I don't think that there is anything wrong with the exposure in the technical sense - I think it is just that photos with such flat, unflattering lighting tend to be rejected because there are so many alternatives of the same aircraft taken in much better conditions.
Generally I think you just have to accept that there is not really anything you can do to make this kind of image 'better' - because there's nothing at fault with it. It's the lighting that's counting against you with regard to successfully uploading, in my opinion.