I need your help before I can go to bed - go to this interview tomorrow or not?

If you are almost positive you would not take the job, and you have an ideal plan, then you must mean you are afraid of burning bridges by not going to the interview or having them think you are not interested in working there. There is nothing wrong with cancelling an interview. It would be professional to ring in the morning and officially lodge your withdrawal. That is considered an acceptable thing to do as peoples plans change, (rather than just not turn up), and would therefore not burn any bridges. You could apply for new jobs there anytime in the future without guilt. They won't remember you cancelled before and it won't matter anyway.