Hi, we use
Acorn Web for our big books. Whilst I have little to do with the company's print buying, I do believe they're quite reasonable but are very strict about deadlines and page counts. However, I am also sure you'll be able to find a competitive quote from a forum member.
Just out of interest, Where abouts in the country are you based? Do you have someone producing adverts for you? This is what I'm currently doing day in day out so if you'd be interested in outsourcing some of your design work please feel free to get in touch. Having an 'in house designer' is a great selling point! (shameless self promotion finished).