The US postal service has free boxes to use with Priority or Express mail. You can order them on their website here:Where do you buy/get your boxes from? i had a look on the internet and they're really expensive!
http://shop.usps.com/webapp/wcs/stores/ser...amp;WT.ac=13354
I realize you are in the UK, and therefore this wouldn't be an option for you. Just thought I'd let those in the US who didn't already know about it. Maybe the UK has a similar service?
Otherwise, you can save appropriate sized and strength boxes that you get when you receive things in the mail. Or ask your local grocery store, liquor store, etc. They may have good boxes to use that they just throw in the recycle bin.