First make sure you have all the necessary components installed—Agent core components, the SAPI runtime binaries, and a SAPI4 compliant British English TTS engine like the TTS3000 British English engine, available for download on the Agent Downloads page. If your character is still not using the British English TTS engine, it's likely you also have an American English TTS engine installed. Since both British and American English share the same primary language (English) and American English is the default, Agent will pick the first available American English TTS engine as returned by SAPI. To use a British English TTS engine, use the character's TTSModeID property instead. For example, the TTSModeID for the TTS3000 British English male voice is {227A0E41-A92A-11d1-B17B-0020AFED142E}. In Microsoft Visual Basic you can use this engine by setting Merlin's TTSModeID as follows:
AgentControl.Characters("Merlin").TTSModeID = {227A0E41-A92A-11d1-B17B-0020AFED142E}