UPDATE (7/02 3:30 pm PDT; 10:30 pm GMT): The problem has now been resolved and all Mail UK users should now be receiving messages from Yahoo! Groups normally, including the backlog of messages that had not been delivered earlier (if you are still having delivery issues, please contact Mail Customer Care). Our apologies for the inconvience.
We’re aware of the reports from Yahoo! Mail UK users that they are not receiving messages from their Yahoo! Groups and both the Groups and Mail engineering teams are investigating the problem. Will update this blog post with more info as we have it.