I used to send bumpers all over the world, just wrap it up in bubble wrap and paper, and make sure you take it to your main post office, not just any side street office otherwise they will laugh at you and send it global priority (not economy!)
Or just do it online through parcelforce or someone similar and they will collect it from your house, you may have to fib about the size though, but the drivers are usually sorted and take anything
I had a brand new front mk4 bumper from a dealer and it came in a big fat box. However, it was broken by the stems that curve up for the headlights. They eventually got me another one which arrived ok. The front bumpers are incredibly floppy and fragile.
I'd try and get a box from a dealer part room, or a local bodyshop. Perhaps ask them not to throw one away.