I’ve noticed Kate tends to wear designer if she is visiting and institution, like a museum.
Out doors, definitely her outdoor gear.
If it’s people, she will wear a relatively i expensive dress in a muted colour navy / wine / dark green with a conservative print, eg polka dot.
So using these criteria, I think she tends to go for high end high street; usually LK Bennet or Hobbs. Other options which I have seen in a while or not at all are Reiss or Whistles.
Kate likesa conservative dress with below the knee and high neck so there are no upskirt or down cleavage shots. Also tends to wear silk.
So I’ve had a quick look online and they all seem to be lots of black; which she only tends to wear in the evening and party dresses on sale; so I am curious to see what she picks.
I really think it’s going to be an inexpensive dress as the charity is to help familiar so she will be smart, but not in your face head to toe designer; I think .