Is it reasonable to say I'm not happy at my current position at interviews? I am working as a research fellow at a University in UK. Its a two year contract with no chance of extension or renewal. Although I have 1 year left, I have considered not staying for the second year, and I have started applying for jobs elsewhere. I have been called for an interview at another much higher ranking university.

They will most probably ask why I am leaving my current position. And I don't know how best to answer this, as I don't want to say too much at this stage.

The University I am currently employed at is not purely research targeted, so along with our research targets we are given modules to teach along with assignment/exam checking. This takes a lot of time from my research. 

Also I am personally not satisfied with the quality of my work here as I have been told quantity over quality and I am producing subpar papers at least once a month and even after discussing with my PI to target better journals he still expects me to continue working and printing out papers, some of the papers that I have produced are so bad in such low ranking conferences/journals that I am ashamed to add them to my CV.

Also as mentioned above, this project has no chance of extension, but is this a valid reason? Would this not alarm my new employers that this guy might leave us as well as the project comes to an end?

The place I have applied for is known for its research excellence. And I really want to move there. So, based on the above reasons how should I answer this question.

Also I am not ashamed in admitting that I am very nervous as I am really hoping to get this job. I don't know what to expect at the interview and how to prepare for it. I have found out the name of the PI and have read his previous published work. Is there anything else I can do that would improve my chances of succeeding at the interview?

I have also read this answer, and was wondering if anything else can be added to this.

Thank you.
A traditional interview technique is to reframe such questions.  Don't focus on the fact that your current job is bad - view it as an opportunity to discuss how the new job will be better and how you're a good fit for it.

Bad: "In my current job, I have to teach too much, and good research is not appreciated."

Good: "I'm looking for a job with more opportunities to pursue research, in areas where I can make the best contribution.  I have successful experience in doing X, Y, and Z, and University A is known for its excellence in this area."