I'm fairly sure that NJ is correct (which, given the nature of his occupation, he ought to be anyway!).
However I'm wondering whether a skilled barrister might argue that it was an implied term of your contract with the theatre that the band would be supported by an orchestra (especially if such accompaniment was mentioned in their advertising for the show)?
If so, that barrister might then direct the court's attention to para 11, Schedule 1 of the Consumer Rights Act 2015 which states that "a term which has the object or effect of enabling the trader to alter the terms of the contract unilaterally without a valid reason which is specified in the contract" may be regarded as unfair.
Then, unless the theatre's small print specifies a valid form of reason for altering the contract (which embraces the absence of a supporting orchestra), it might be argued that they have broken the terms of the contract.
However, despite of the foregoing, I still feel that our imaginary barrister might be clutching at straws a bit!