Regardless of needs I'd say it's discriminating against wheelchair users, if able bodied people can sit together why do those in a wheelchair not have that right? I'd bet none of their pre-booked audience who were not wheelchairs users were not moved. I reckon it is a situation covered by the DDA 1995 in section 20 (relates to sale of goods or services:
(1) For the purposes of section 19, a provider of services discriminates against a disabled person if?
(a) for a reason which relates to the disabled person?s disability, he treats him less favourably than he treats or would treat others to whom that reason does not or would not apply; and
(b) he cannot show that the treatment in question is justified
Your DH was treated less favourably than other audience members.
Further:
Goods, facilities and services
19 Discrimination in relation to goods, facilities and services
(1) It is unlawful for a provider of services to discriminate against a disabled person?
(a) in refusing to provide, or deliberately not providing, to the disabled person any service which he provides, or is prepared to provide, to members of the public;
(b) in failing to comply with any duty imposed on him by section 21 in circumstances in which the effect of that failure is to make it impossible or unreasonably difficult for the disabled person to make use of any such service;
(c) in the standard of service which he provides to the disabled person or the manner in which he provides it to him; or
(d) in the terms on which he provides a service to the disabled person.
By separating wheelchair users and carers they make it unreasoanbly difficult for wheelchair users to access the service they provide.
Quote this at them and explain you either want an apology and the situation rectified or to take the matter further.