I'm finding tickets available on all of the random dates that I've just checked. (Obviously some dates will have more tickets available than others, with the cheaper tickets often being the hardest to find but still with some £19.50 tickets occasionally).
https://www.wickedthemusical.co.uk/book-tickets/
(The theatre's accessibility policies can be viewed by clicking the link in the left hand panel on that page. There don't seem to be any specific discounts for disabled patrons or carers).