Snapchat messages being surreptitiously downloaded and monitored by the NSA at some point where they are unencrypted (possibly only within Snapchat's datacenters) is consistent with what was revealed in Prism. I know that most companies have updated their standards to treat any network traffic as insecure since Prism but it's possible Snapchat doesn't.

Also as I understand it the NSA also has no legal issues with sharing any messages intercepted with the UK so long as one of the parties to the conversation is not a US national or on US soil.