Can I use the M12 fitting on the front of my mk1 Fiesta brake master cylinder to fit a pressure switch to trigger the brake lights?

It's the same M c as shown on this thread www.locostbuilders.co.uk/viewthread.php?tid=188982

I don't see why not, I fitted a blank.
The switch is a banjo fitting on one of the rear outlets, photos in the thread you linked