This confirms my comment that PrinterPix don't know what they're doing:
http://www.trustpilot.com/review/printerpix.co.uk
I've used Costco's photo services with no issues to date and they have Printer profiles you can use with LR's Soft Proof tools. The profiles are for specific printers by store location.
http://www.drycreekphoto.com/icc/
The Costco photo services manager was more than happy to discuss how to prepare and upload JPEGs to insure the best quality. Unselecting 'Auto Correct' during the order placement directs your image files to the Dry Creek profiled printer. There are two standard printer profiles and two poster printer profiles. Make sure you use the correct profile in LR for your soft proofing based on 1) enlargement type and 2) paper type.
Costco Dry Creek Printer Profile Instructions can be found here:
http://www.drycreekphoto.com/icc/using_printer_profiles.htm