Hiya
I've already had my interview at Gloucestershire, and have a conditional offer from them (even though I honestly didn't think the presentation went that well.) So if you're feeling least confident about that part, just work on the interview skills, because they like you to evaluate your own presentation, and say what you would improve if you did it again
Try not to worry about the actual object itself, as long as you have a really good reason of how it would be useful in a classroom, and if you have seen it used in the classroom before, it should be alright.
It's always good to expand on the object, and explain how it could influence projects including different national curriculum subjects.
E.g. if you took a roman artifact (completely random example) - you could say how you'd use it to develop research skills and also use for the history, then they could draw a roman house (e.g.) and then read a bit from a book, and act out, pretending to be Romans.
This is a completely random example, but what I was trying to get at, is just make sure you can incorporate a variety of curriculum subjects to the object, and explain how it would encourage learning.
Hope this helps a bit? Good Luck!