I have to implement this in twenty two bathrooms. Can somebody suggest a solution to this?
The nuts leading to the mixing valve is what is keeping it on the pipes. Undo those (after shutting off the water) and the mixing valve will come off.
Then you can install a new tap assembly. There are models that come with a diverter where a shower can be attached to. Those are often used for bathtubs.
That is an "external mixing valve" the advantage of which is that the wall does not have to be opened up to change the entire mixing valve. Look on youtube for videos showing installation of external mixing valves.
The usual expected spacing for the supply lines for an external mixing valve is 15 cm (about 6 inches). Is that what you have?