In 2010 the disability & equality act 2010 replaced most of the DDA, the disability equality duty continues to apply. You may well qualify under the disability discrimination act 2005 18 (2), Under section 3A of the disability discrimination act 1995, it states:
“ A person discriminates against a disabled person if for a reason which relates to the disabled person’s disability, he treats him less favourably then he treats or would treat others, to whom that reason does not or would not apply. “
You are saying your employer is insisting all (not some) employees act as if in a sales capacity, so he is not discriminating against you. You claim this has a more harmful impact on you due to your condition, so I would strongly suggest you ask why your employer is acting in this way, is the company in trouble? At the same time explain why this has a greater impact on you than it might others. There is a lot to be gained by talking over things that affect you with your employer; to try the legal route may not be the correct one straight away and may be very difficult