The only way to get a refund for ticket is if the performance has been cancelled for any reason. However, West End theatres can attempt to resell your tickets for you. You will need to return the hard tickets to the box office. If the theatre is successful in finding a buyer for your tickets, you will be refunded for the cost of the ticket minus the booking or administrative fees.
You can also pay an extra £1.99 for Flexiticket, which allows you to change your booking or receive a voucher up to 3 days prior to the originally booked date.