@joshd , CAB is the Citizens Advice Bureau. You were right to pose this question as we assume that it is a known acronym.
The face-to-face assessment is just a “tick box” exercise. They have to check the facts about what you can’t do.
Yes, you are quite correct that arriving alone does tend to undermine your “can’t plan a journey” argument. So , you need to plan an appropriate response in advance, e.g. on a bad day, you would have asked a friend/family member to take you.
The more documentary evidence that you can supply, to support your case, the better.
I recently applied for PIP on behalf of my Brother-in-law, whose suffering “cognitive impairment”, i.e. he’s presently “losing his marbles” and is in a dreadful state of not being able to do much for himself. The application form was completed objectively with the “can’t do’s” and was accompanied with copy correspondence from Counsellors, Neuros, Dementia Assessors, etc..
He had a face-to-face assessment at home and was unfortunately able to “play a blinder”. He was awarded enhanced care AND enhanced mobility, without need for Mandatory Reconsideration. He now gets more than me and I’m housebound!! (To be fair, I haven’t updated the DWP about my condition since it was granted in 2005. My MS has progressed markedly in that time!)