I would charge at least, if not MORE than, what you'd earn on an average day currently.
It's going to be hard work. I used to work for the NHS in a MH unit, it's a faff getting in and out, if it's anything like where I worked you'll need to book all of your equipment in and out and account for every single item down to your last hair grip at the end of the day.
It's unlikely to be a day of nice washes and blow drys, You're probably going to be fixing up some hair that's in some terrible states - think along the lines of unwashed and not brushed for weeks.
You'll be provided with a list of restricted items, you should be accompanied by a staff member at all times and they'll talk you through personal safety, if its for the NHS you might even have to attend their personal safety course.