The Environment Agency (EA) has its own bespoke XML-based schema for its FWD, FWIS and Floodline systems. The FWD standard follows the usual XML format consisting of a declaration, a root element and child/sub-elements. It differs from the CAP standard in the terminology used (being more specific to the FWD and FWIS systems) but uses the same Base64 encoding for embedded resources. The EA acknowledges the ease at which the EAFWD schema can be converted to a particular style of CAP XML but highlights how CAP XML is not specfic enough to improve interoperability over existing systems. The possibility of extending the EAFWD schema to fit a more generic national alert system could be worth investigating as FWD already generates content for mobile SMS messages.
The Sesssion Initiation Protocol (SIP) has been adopted as a standard for inter-operability between telecommunication platforms, and would potentially be appropriate to the challenge. However, whilst the addressing and payload of a SIP message is relatively straight-forward (not dissimilar to email) SIP is a relatively comlex protocol and may not be readily familiar or accessible to smaller system vendors wanting to integrate their user-interface/platform capability within the framework.
From a perspective of the raw message syntax, the two obvious choices would be XML and JSON; both are human-readable, extensible, highly adopted and readily available (i.e. non-proprietary and without license charge). UK government has selected XML as the preferred standard (e-GIF) and so it would make sense to base the UK standard around XML.