I'm a HoD for Science, albeit not independent, so hopefully my advice helps.
There is nothing wrong with doing a demonstration as part of your lesson, just some things to consider (which I'm sure you would anyway!)
*how long is the lesson? If its a full hour or 30 minutes +, you will be fine. Our school gives 20 minutes and you would be pushed for time.
*Make sure you trial it before the day - nothing worse than it not going as expected!
*How does it link to the intended learning for the lesson? This is really key. If it is clearly linked to the learning and enhances that for pupils, it's fine. So often though, I see people use a demo which either doesn't clearly link or doesn't add much value.
*What will the pupils be doing while you are doing the demo? They need to be actively thinking/engaging so consider how you will do this.
*sounds obvious but will they all be able to see clearly?! I had a candidate do a demo a few weeks ago with the class gathered round a side bench - about 2/3 couldn't see clearly and some at the back just lost interest. If it is something large it will be fine, otherwise, ask if there is a visualiser to use.
In a nutshell, Yes, demonstrations are fine in interview lessons but they need to be well planned.
Best of luck!