As above what is it on my Mondeo titanium? What does it do and why is it always unavailable 

Do you mean the driver alert monitor that uses a coffee symbol?

If so, it’s intended to tell you when the car thinks you’re showing signs of fatigue and should stop for a break. 

As okay it says unavailable always though or maybe I’m never tired 

Unavailable would suggest whatever sensor it normally uses has a problem so the system isn’t working.

or it knows you're a tea drinker and no point in telling you to stop for coffee 🤣

It uses the lane keep assist to detect wandering over lines. Too many times and it warns you, also it's on time at the wheel driving. It's triggered on me a few times on long journeys. If you constantly overtake without indicating that triggers it. That feature is obviously not on BMWs and Audi's. 

I think it can be switched off in settings menu so it might be switched off.