It's too expensive. Others in your block are available for 25k+ less - no, they're not as well decorated, but they are the same size or even bigger, and a bit of paint will get them looking just as lovely as yours currently does. You can't expect nicer than average decor to justify such a huge price increase.
It's a lovely, light and airy flat, and you've made it look beautiful, but the reality is for that price you can get something of a similar size in a much more central area. I'm a lifelong Londoner and have never even heard of Highams Park - I had to look it up on a map. It's not got a tube or truly easy public transport access - the Overground isn't that reliable- and it's a bog standard new build flat on an otherwise very uninspiring street. To be that far out of central London or away from convenient public transport, I'd expect something truly fabulous for £425k - a private garden or at least a terrace - and your balcony is small. There's also not a particularly nice outlook, which would be what you'd be moving to suburbia for - trees and such from your windows.
On a practical level in terms of things you can change to make it more desirable, as others have said, you've got a big waste of space in that hallway - I'd definitely move the desk into that space and turn it into an office so that it becomes a room in its own right that allows viewers to see how much useable space is on offer. I'd also remove all the extra bits and bobs on top of the kitchen cabinets - nothing says 'this kitchen is too small' more than cluttered cabinet tops and surfaces.
I should think if you went down to £400k you'd get much more interest. It depends how quickly you want to sell, though - the summer is always slow.