How else would excess water from a dripping ball valve escape if not
through an overflow. How does the other type of cistern work which
doesn't need an overflow? Is it possible/legal for the overflow to
drain into the normal outflow from the cistern to the pan? If it is,
I'm not sure why all cisterns don't have this sort of overflow.



Clearly you are out of date. Most modern cisterns overflow into the pan and
do not need an external overflow (or Warning Pipe as we were told to call
them). Most cisterns have a cut away area at the back so that if the
overflow is blocked the water will run down the wall - I think it serves to
prevent people setting the water level too high.