Hi there,
Speech and Language Therapist here
Did you have to fill out any info on the job application where you could let them know you have a stammer e.g. under health? It might be a good idea just to let them know either before hand or at the start of the interview - not just to make sure that they don't interrupt you (which they shouldn't be doing!) but for your own confidence/reassurance. They should not be able to discriminate against you due to the Equality Act 2010 and should allow you extra time if you need it. This is not a reason for you not to get the job, especially since in day to day life you're managing it.
Remember that even people without a stammer really struggle in interviews too e.g. they might talk too fast, run out of breath for their sentence, voice may be shaky. So no interviewer is going to expect complete confidence or fluency (which doesn't exist).
It's great that you've been preparing. Do you know if you can take any notes in? If you got really stuck could you write down the word?
I'm not sure if you're asking for specific stammering advice or just general advice? But for specific stammering advice... what type of stammer you have? Is it repetition of the first sound of a word, elongating the sound, and/or blocking on the sound (where the sound doesn't come out)? My main tip would be, if you get stuck on a sound, try and think of the next sound, rather than trying to repeat the sound/word again e.g. if the word was 'because' and you were saying b, b, b - try not to start from the beginning of the word with the b sound, but think of the next sound e.g. the e sound so b, b, -ecause. rather than b, b, because - this way you move on with the word. You can also slide into the sound by making it a softer sound. Obviously it's hard to demonstrated this on here - but if you feel that you may need any further support with your stammer then do contact your GP or see if you can self-refer to your local adult Speech and Language Therapy service
Let me know if you have any questions at all!