I am a long-term expat, and recently drove a hired car in England for the first time for a number of years.

Not only was there no handbook or even minimal instructions in the unfamiliar car, there was no list of standard speed limits. I did not see any indication thereof on leaving Stansted airport, and was left wondering for my entire stay.

An “unrestricted” sign is meaningless - does it mean what it says, or is it related to a limit about which I knew nothing? What about dual carriageways and country roads outside built-up areas? The plethora of constantly changing speed signs in restricted areas is confusing, as you rightly say, but a sign on entering a motorway or dual carriageway (hard to identify the difference) would help foreign visitors, and perhaps serve as a reminder to residents. In France, there are frequent reminders on all roads - why not in UK?

There is certainly no way of guessing by the behaviour of other drivers in England, which seemed totally anarchic.