For this test we used Photobox’s Poster Prints and Pro Poster Prints, printed on Fujifilm Crystal Archive paper in a gloss finish, as this was where the sizes we needed sat in the range. We found the site straightforward and easy to navigate. You first need to sign up for an account, but make sure you unclick Optimize Image in the Preferences to avoid any colour cast on your black & white prints. From here on, you choose the size before you upload and will then be given options to rotate or adjust any cropping and a warning will appear if your photo is too small for the print size chosen. 
Within three days, we had our prints. They arrived in a roll tube, protected with packing paper to stop movement and a covering of Photobox branded paper to protect the prints, too.
We noticed on the 16x20in print that the cut was not flush (there was a millimetre or so of white edge on one corner), but the print quality itself was decent, with no colour cast. Blacks were deep and rich, and the mid-tones maintained a good level of contrast, but the highlights suffered a little, and began to lose detail.

£27.71 for 3 prints & postage