To address the "pure aesthetics" aspect of the question, there was some but not much experimentation with language (amd Stoppard can be cited here, as well as Pinter), the theatre language underwent some aesthetic changes--the relation of the audience to the fictive imitation of an action, the alteration of the well-made play structure, the symbolism of light, costume, sound aesthetics, and the expectations of the audience regarding closure. But the major aesthetic change was the two-act play structure instead of the three-act play structure assumed in the 19th century. The two-act play structure, much more than a change in the number of intermissions, called for a significant change in circumstance or mise-en-scene, a distorted mirror image, a significant relation between the first and second acts, that could be considered an aesthetic adjustment.
Twentieth-century English drama, being a 100-year period, includes a variety of writers who address many different themes. Although, for example, George Bernard Shaw was concerned about a variety of social issues, his plays are very entertaining comedies. Pinero and Coward wrote light drawing room comedies, which aimed at being entertainment rather than social commentary. The Angry Young Men, such as Osborne, wrote working class dramas concerned with social problems and conflicts. Beckett and Pinter both wrote dark plays about the human condition, but were not really social problem writers. Tom Stoppard's plays are intelligent, thoughtful and often funny, with more of an aesthetic and intellectual than social problem focus. It's not really possible to generalize about themes over the entire century.