What is reasonable will vary depending upon your employer and the effects of your disability. Your employer might consider:
how much difference the adjustment will make to you
whether the cost is affordable for them
whether the adjustment is practical
whether the health and safety of others could be affected
If your employer is refusing to make adjustments that you believe are reasonable, you should first raise the issue with your employer either informally or as a grievance (though it will not prevent you making a claim if you do not). If they still refuse, you may make a legal claim.
You may claim against an employer you no longer work for if you believe that reasonable adjustments could have helped you to keep your job, so long as you are within the time limit for making a claim.