Steve
Based on your post they may just move the appointment to a centre without steps.
If you want a home assessment then you will need your GP to write to HAAS explaining why you cannot attend at their offices and why, the letter must be clear that you cannot travel or that it will have a detrimental effect on your health. Any suggestion that you can travel or that there is a risk of danger to you rather than a certainty, will almost certainly invalidate the letter.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems