I've done only one exhibition and I used Photobox. Their larger prints are excellent. I did try to use Loxley, but there were too many logistical problems (I won't go into details, but it didn't involve print quality), and I couldn't get the printing done in time. Luckily Photobox were lightening fast.
Loxley do have a fantastic range of products and are very flexible, so if you have a bit of time to get your head around their system I would definitely look there first.