We have confirmed that Three UK is having issues sending text messages. This is intermittent, with some messages going through quickly and others not yet making it through after an hour or more.
We would expect that the queued up messages within the Three network will be delivered as the issues are resolved, but cannot guarantee that. Other mobile networks are not affected and for context we understand that Three UK has about 12-15% of the UK share for mobile phones.
We will update you here as we learn more. If you wish to check status for Three direct, you can do that here: http://www.three.co.uk/support/network_and_coverage/network_support