I would suggest that you ring the Assessment Centre to ask if they have a lift. If they say there isn't then you could request that the Assessment is carried out in a ground floor room. I believe that Walsall Assessment Centre has Assessment rooms on the ground and first floors. A point to remember is that in the case of the building needing to be evacuated you would not be permitted to use any lift.
Nothing on this board constitutes legal advice - always consult a professional about specific problems