A licence is required where it is intended to perform any play in public. It does not matter if the performance is free, a licence is still required.
'Play' is defined in the 1968 Theatres Act as “any dramatic piece, whether involving improvisation or not, which is given wholly or in part by one or more persons actually present and performing and in which the whole or a major proportion of what is done by the person or persons performing, whether by way of speech, singing or action, involves the playing of a role”. This includes any ballet.
A theatre licence is required for any premises or any place whether or not another type of licence is in force.