Apple's dictation service, introduced in OS X in 10.8.5, allows you to speak phrases and have the system enter them as text in supported text fields in most applications. This required you to be connected to the Internet so recordings of your voice could be sent to Apple's servers for the conversion process. This changed in Mavericks with the option to enable Enhanced Dictation, where the conversion tools are downloaded and run locally to give both continuous feedback and the ability to work offline.
In some instances, though, you may find the Enhanced dictation feature to perform less accurately than the non-enhanced version. This may be from the enhanced dictation needing to learn from you alone as opposed to learning from the feedback of many who use the service, but there also may be a few bugs with the service that can prevent it from working as expected.
For example, you may find that the service is working well and then it suddenly starts making mistakes (this may happen more if you are using a non-US English language with the service).
There are a couple of things you can do to fix such problems if they show up, the first of which is to try clearing the cache for speech recognition. To do this, first disable dictation in the System Preferences, and then go to the Finder, open the Go menu, and hold the Option key to reveal the link to the user library. Select the library, then go to the Caches directory and remove the folder called "com.apple.SpeechRecognitionCore." Following this, re-enable the dictation service.
Unfortunately, for UK English in particular, there appears to be a caching issue where a misspelling has been found in the cache files, which may be the root of the problem with this language pack. Therefore, even though clearing the cache should clear the problem, it may resurface again.
While this may be a bug Apple needs to address, one workaround for now is to use a similar language if it will suffice, such as US English instead of UK English, and then simply account for differences in spelling that are used (e.g., "color" instead of "colour").
The next option may be helpful if you can't get enhanced dictation to work at all. You can clear the downloaded speech recognizer and then have the system reinstall it. To do this, first uncheck Enhanced Dictation and disable the Dictation service, and then go to the System > Library > Speech > Recognizers folder and remove the folder called "SpeechRecognitionCoreLanguages." You will need to supply administrative credentials to remove this folder, but when done, go back and re-enable Dictation and then activate Enhanced Dictation to re-download the service.