Not necessarily. Most adjustments made by employers are low-cost or even free.
The Equality and Human Rights Commission (EHRC) advises discussing the adjustments with the disabled worker, otherwise the adjustments may not be effective. It also points out that what is reasonable depends, among other factors, on the size and nature of the organisation. It has further information on workplace adjustments on its website.
Under the Equality Act, it is unlawful for an employer to pass on the cost of making a reasonable adjustment to the relevant worker (unless the contract expressly permits otherwise).