What puzzles me is what triggered the need for an assessment as I assume you have already been driving as a disabled person.
All I know of assessment centres is back when I became disabled holding a full license for manual cars I was asked to pay £75 upfront for the assessment unless I’d been referred to them by DVLA. DVLA just changed my license from manual cars to automatic only with adaptions and that was that with no assessment needed.
Had as much info about driving in my condition from my local car dealer as I did from the assessment centre but sounds a bit frightening to have something come out of the blue demanding you need an assessment without no cause for concern in ones driving habit.