When scanning, how frequenty should you take your eyes off the road for a couple of seconds? That's to re-focus on the digital display, read the numbers (ugh, but that's the modern way), re-focus on the road.
A lot can happen in a couple of seconds.
The solution to that problem, if it is a real problem, is a heads up display that reproduces essential dashboard information where it's easy to see. In fact many modern cars have this, either as standard or as an extra cost option. There are third party heads up display solutions available. Some dedicated, some that include mounting a phone so that it reflects a speed indication off the windscreen.
In practice glancing at the speedometer doesn't take 2 seconds, more like 200 - 300 ms and you only need to do so if you've substantially changed your speed upward, so much so that you can't internally estimate whether you're still within the speed limit. One glance at one's speedometer every 30 seconds will consume 1% of one's attention, the other 99% can stay on the road.
As it is, my own experience is that I can largely ignore the speedometer and set my speed by the conditions and later review of dashcam recordings complete with GPS derived speed show that that I'm automatically keeping below the speed limit (without any real audio cues; my current hybrid car is very, very quiet to the point that I often only notice that it's switched the petrol engine on by the position of the rev counter needle). On most urban roads my speed is often much further below the speed limit than one would think; limited mostly by the hazards that are in view, I rarely travel over 25 mph.
In the UK knowing what the current speed limit is, is trivial. You're either on a lit urban road where the limit is 30, a national speed limit road outside town where the existence of a central reservation tells you whether the limit is 60 mph or 70 mph. For everywhere else there will be clear speed limit signs by the side of the road where limits change, with repeater signs every 200m or less for 20 mph limits, scaling up to repeater signs every 600m or less on 70 mph roads. The first repeater sign after a limit change will be at most 200m for a 20 mph limit and 450m for a 70 mph limit.
Thus if you're in any doubt what the current limit is on a posted road you have an average of between 100m (11s) and 300m (9.6s) until you
pass the next repeater, therefore absolute maxima of 22 and 19 seconds for 20 mph and 70 mph respectively. Most likely one will be able to see and read it sooner.
With all that in mind I don't think there is any excuse for not knowing the current speed limit, and to my mind any justification that one needs a speed camera warning system to avoid inadvertently exceeding the speed limit thus fails. All that is required to avoid prosecution is to (1) pay sufficient attention to what the limits are, (2) sufficient attention to what one's speed is, and (3) sufficient attention to bright yellow boxes stuck on posts at the side of the road with the warning signs that precede them. If one is already paying sufficient attention to the many hazards on the roads, taking in those three bits of information is trivial compared to the unpredictability of other road users.
Speed cameras don't catch speeders, they catch inattentive drivers and I'm all for that. It's not speed that kills, it's inattention.