Intelligent speed assist will not become mandatory in the UK as the previous government opted out. However, your vehicle may already be fitted with “intelligent speed assist” as standard or as part of an option pack (my new vehicle will be), in which case it will be on as a default setting each time you start your car. In this case you can “legally” delete it before moving off by pressing one or two buttons (this will no longer be legally allowed in the rest of the EU).
There is a a difference between “advisory speed assistance” and “intervening speed assistance”. ‘Advisory’ is just an audible reminder that you are exceeding the speed limit, and ‘Intervening’ actually slows the vehicle down to the speed limit if you ignore the audible warning.
I am a slow driver so it didn’t bother me at all when I had a test drive in a car which had it fitted.