Nothing will change, this is a bilateral agreement between UK and Germany. Others are in the pipeline.
Nope. As per EU directive 2015/413 all member states exchange registration data and can use it to forward you any traffic violation in excess of 70€. Below that they seldomly bother to send you a ticket.
So after Brexit there should be a new bilateral treaty but if/when this happens.. well see.
Therefore while driving through Germany you should try to limit your speed to a max. of limit+30kph according to the speedometer and the German Bussgeldkatalog:
https://www.bussgeld-info.de/bussgeldkatalog-geschwindigkeit/ 
