It seems that they are in the process of updating the expiration dates for the 2-for-1 vouchers. This morning when I looked, Westminster Abbey had been added with an expiration date of October 31st, but the Tower of London was still showing an April 30th expiration. I just looked again and the Tower has the October expiration now, but some of the other ones are still April. I just wanted to mention it since there have been questions about when these updates would be done.
Happy travels everyone!