It's very much a personal choice. I would tell them about your diabetes, because if you don't tell them, and they subsequently find out (which they almost certainly will), they may think you've been dishonest. That's the last thing you want when you're working in a shop and - presumably - handling money. Similarly, if you have a hypo and you're going to be late for the interview, I'd let them know about it and explain why. That's far less likely to prejudice them against you than if you turn up late without any explanation.
When you go for interview, I'd just say to them that you need to explain that you have Type 1 Diabetes, that generally speaking it doesn't prevent you doing anything, but that you need to check your bloods from time to time and sometimes inject insulin (or are you on a pump?) for which you'll need a bit of privacy, but that in general it doesn't prevent you from leading a perfectly normal life. Maybe also explain that you may occasionally have a hypo, explain what happens and what they might need to do, offer to talk it through with their first aider (who will have been trained about it anyway), etc. I would say that the more you make it clear that you have thought it through, and tried to inform them about what they need to know, the more likely they are to be impressed by you. It's also clear from some of your previous posts that you love animals, and that's got to be important as well, I'd say
Before all this, you need to ring them up and arrange the interview, so get on and do that
Good luck.