Every building has a practical limit for the number of people that can be safely accomodated, able or disabled. This upper limit may be specifically stated in the fire certificate or by other people eg the Licensing Authority for places of public entertainment. This limit depends on the construction of the building, ie the number of exits available, alarm system, refuges for the disabled, etc.. If there is a limit on able people, then I see no problem, legal or moral, on limiting the number of disabled people to that which the building can safely accomodate at one time.
After all, if you let too many wheeelchair users in and there was an incident in which one or more such people were injured, perhaps because there was insufficient refuge space, I have no doubt the lawyer for HSE and for the victims would pounce. I'd rather answer a lawyer with safety reasons for refusing entry than one suing for compensation because I let too many in!