I'm pretty sure mycroft sends your speech snippets to Google for processing so it's not exactly offline.

https://mycroft-ai.gitbook.io/docs/using-mycroft-ai/customiz...

I'm currently trying to setup a deepspeech server on my raspberry pi to see if it works ok for commanding spotify.

Edit: just realised you said `TTS` not `STT`

pico2wave with "-l=en-GB" option to get the British lady voice is pretty decent (way better than the other voices it does for some reason).