Most cars produced to day (virtually world wide) make extensive use of computer controlled features. This is almost a de facto situation, becasue when a car is to be exported to a different market, there are often differences in the local construction and use regulations the car needs to conform to. There are a couple of fairly obvious ones, the way the road lights operate such driving lights on or off. Turn signals and dimming of other lights, and probably many other less obvious requirements, all of which can be achieved by software. It makes sense to build the car with as many common parts as possible, but achieved the required compliance by installing a software version for the designated area of use.
Some manufacturers now offer software updates to refine the functions of vehicles or allow the driver access to previously locked out functions. These could be options that were waiting for local approvals, or it can be aftersales added options.
With the most recent models, many already have adaptive cruise control which has access to braking and acceleration systems, its a minor step to add the necessary GPS signals and data bases (Which some cars already have) to enable automatic speed restrictors that adapt to local speed limits.
For several years now at least two major players in the Sat Nav market have had stand alone units that signal when the driver is exceeding local speed limits based on an internal database. HGV drivers have had to manage with mechanical speed restrictors on their engines. It's not a major step to have speed restrictors on all vehicles.
Rather than bemoaning the speed restrictor, perhaps law abiding citizens should be pointing the finger at those anti social drivers who think they are above the law, and who consciously are prepared to commit to speed.
Regarding the concept that all EU derived legislation would be cancelled when Brexit happened... think again. The UK (when it was part of the EU) was actively participating in the construction of some the legislation, and it's highly unlikely many of those pieces of legislation will be withdrawn. I see a couple of new posts have been made since I started this, and I agree with both.