Reasonable adjustments
The duty applies to charities as employers and as service providers, and the two operate differently.
As an employer the duty is triggered by knowledge of the disability and the disadvantage. As a service provider it is anticipatory — you must consider adjustments in advance rather than waiting to be asked.
What is reasonable depends on the size and resources of the organisation, so a small charity is not held to the same standard as a large one. ACAS runs a free helpline.