I've experienced a similar problem when sending texts to a friend in Italy. The problem arose because I (like many others) use pay-you-go 'bundles' (of calls, texts and data).
Such bundles are only valid for calls and texts within the UK, so you need additional 'plain' (i.e. 'non-bundle') credit on a phone in order to make overseas calls and texts.
Perhaps you're in a similar situation, where you've got an active 'bundle' on your phone (allowing you to use your phone normally for calls and texts to the UK) but you've run out of 'plain' (i.e. 'non-bundle) PAYG credit?