I've used Ka-Blam for the past 3 years, and havent really had any issues with them. I'm at a point where I printed up a lot of books(I have 2 comic series that I've been working on), so I'm selling what I have before I order more...
I've used Greko Printing to do my "premium" projects(the print versions of my webcomic fall into this category coz they are under/over 40 pages); LOVE Greko's cover stock- a little heavier than Ka-Blam, but captures color pretty well.
Havent had books printed through RA Comics Direct, but I have all my prints done through them. I love their turnaround time and their deal is pretty sweet(75 prints for $50 bucks)
Being in the UK, the shipping is gonna get you(expensive).