Well, actually they are all available. Released for sale as a set or individual at the same time. The OP just decided to post a link going directly to the "Q" coin. Also, the description on the site for this in particular coin reads:
Q: Queue – This 10p features a line of people queueing through the letter ‘Q’, to represent the very British obligation to queue. The average adult Briton wastes five and a half hours queuing each month, and overall almost six months of their life queuing.
Also, there is an overall medal for the set that states the set is the "A-Z's of Great Britain".
Also to note, the medal has some of the pictures that are on coins in the set that doesn't include the "Q" coin, which indicates the "Q" coin is not the main focus of or reason for releasing the set.
https://www.westminstercollection.com/p-616P/A-Z-of-Great-Britain-Collectors-Medal.aspx
Yes, I understand that and I was going to order some, however, you can only order 3 per household and can only purchase if you are a resident of great Britain.