Stacey, brill congrats on getting to the interview stage!
It shows that your CV was exactly the "right stuff" - they wouldn't have shortlisted you otherwise.
I'm a hardened old interviewer in the Brit arena, so I hope the following might help:
1) You will almost certainly be interviewed by a panel/interview board - ie, more than one person.
2) One of these persons will almost certainly be a 'personnel/HR' professional. The reason for this is to make sure that all the organization's policies and pratices are properly understood, followed, and documented. If at some later date you feel that your non-selection was on account of some unfairness, you can appeal to an Industrial Tribunal, which will investigate whether any unfair discrimination took place, according to both the organization's personnel practice, policies, and procedures, and according to English law, both statutory and customary. This person is unlikely to ask you any difficult questions, only to clarify your understanding of what the job entails, and how you will be remunerated for it - this bit is a negotiation or clarification about your emplyoment contract.
3) Other people on the panel/interview board will be your boss-to-be, and probably his/her boss. They will certainly have your application papers in front of them - your letter and CV, and application form if one was issued you. They will probably use these as a 'prompt' in their questioning.
4) The job of the panel is to select the best person for the job. They will take notes, and possibly keep a discreet score sheet (These could be used as evidence in an Industrial Tribunal)
5) Almost certainly, the first five minutes of the interview will be designed to relax you and make you feel comfortable. An interview is a formal discussion, and good interviewers realize that some people do not give of their best in such circumstances. However, they are trying to get the best person for the job, not the best interviewee.
6) Hard questioning starts with one or more of the following questions: a) what are your strengths/weaknesses? b) what do you know about this organization/job? c) what qualities do you think you can bring to this job? It is best to have rehearsed some of the answers to these questions - which you will already will have done, or you wouldn't have applied to begin with!
7) Three or four questions will follow - teasing out relevant experience &c.
At the end of the interview you will almost certainly be asked if you have any questions to ask. If you like, as well as any genuine questions you might have, you can use this to demonstrate that you have done some homework on the job/organization, but obviously don't make your questioning sound too challenging and critical.
Never lie, nor dissumulate at an interview (well, dear Stacey, I know you never would!)
Remember that the interview is only one component of the selection process, and often not even the most important one.
Also use the interview as a learning process, to find out about the organization and the job they have for you on a plate, to find out more about yourself and what you want to do, to find out more about other people.
If you don't get the job, don't grieve: why worry about a job you never had anyway?!
I hope this helps. I know that other forum members will be with me in the pain of their tightly crossed fingers for you! Luck, oddly enough, does count, as in horse racing (your uncle Celtic will back me on this!) , so I wish you tons and tons of it, dear Stacey!
Howard