This is the sort of issue that lawyers can argue over for hours.
As I see it though, if a cinema (for historic reasons)
only had an upstairs screen (and there was no way to install a lift or otherwise provide access for people with mobility difficulties) then a court might rule that there was no 'reasonable adjustment' available to the owners, so that there was no unlawful discrimination involved.
However if a cinema has both upstairs and downstairs screens available then it would seem to me that they should offer at least one showing of a film on a downstairs screen (upon request, if necessary) as a 'reasonable adjustment' to their original plan to use only an upper screen.
Legislation here:
http://www.legislation.gov.uk/ukpga/2010/15/part/2/chapter/2/crossheading/adjustments-for-disabled-persons
General advice here:
https://www.citizensadvice.org.uk/law-and-courts/discrimination/discrimination-because-of-disability/what-counts-as-disability-discrimination/#h-access-to-goods-facilities-and-services
Complaints procedures here:
https://www.disabilityrightsuk.org/making-complaint
However, if the cinema is part of a major chain, I'd suggest contacting their head office in the first instance.