My postwoman does not wear a uniform - in fact she does not wear anything at all - and she looks like Jennifer Ellison. Not only that, but she delivers the post to me when I am still in bed........
as per an earlier answer - they should ALL wear the uniform provided. That's how you know they're your postman.
However, more and more casuals are being used to cover holidays / sickness, and they wear their own clothes.
It's a far from ideal situation.