I was also going to add to be prepared as you sometimes don’t get much notice about your face to face, my last one when my letter arrived I had two days notice, it was a bit of a shock, but I just wanted it out of the way.
you can cancel and re-book once but after that they will make you start your claim all over again.
The other thing I’d recommend is making sure if you are planning to go by cab then make sure you ring and ok it first to make sure you get your costs back.
If your driving you can claim back mileage costs, not parking charges, they will give you a claim form as you check in.