0.7mm pitch and 12 pins means 11 gaps between the centers, hence a length of 7.7mm when measured between the centers of the outer pins, as illustrated on that diagram.
You have to draw it out on paper say at a scale 10:1 so 0.7mm is drawn as 7mm to see what happens if you use a connector with a pitch of 0.8mm. The effect would, to a certain extent, be dependent on the width of the individual contacts but, for the 0.8mm connector, you can assume 0.4mm. I guess, quite soon, these would begin to overlap on your 0.7mm pitch cable. Over a very small number of pins it may work for a prototype but definitely not recommended.
You'll also have difficulty soldering to a prototype board. There the pitch will not match your 0.7mm. You'd have to use fine wires between the cable and the board.
My advice is, for the prototype, work with another display. If it really comes to a commercial product, and the cost of the display becomes an issue, then start looking at solutions for integrating lower cost displays in discussion with the PCB manufacturer (who may also populate boards).
In this area I have no direct experience, but other people on this forum have this background, for example @ Paul_KD7HB