So you'd flick the limiter on, and it would then default to the signs rather than user input?
Yes, that's right. (Actually I'm not clear on when it uses signs and when it uses information from the mapping data.)
Please bear in mind that it is not perfect; the car often makes mistakes over the speed limit. For example there is a long stretch of 40 mph road near where I live where the car thinks that there are a couple of 30 mph sections. So, if using the speed limiter and driving at 40 mph you will find yourself unintentionally slowing down where the car thinks it has entered a 30 mph limit. Although you can override the limiter by using kick-down this is perhaps not always appropriate, and is not necessarily appreciated by any passengers. One alternative is to use the up (or down) arrow to adjust the limit but, particularly if there is traffic immediately behind you, perhaps the better option is to disengage the speed limiter then adjust the limiter setting and then re-engage it.
Rather a long, rambling, post, but the point I'm really trying to make is that you should practise engaging and disengaging the speed limiter so that you can do it quickly and reliably without needing to look at the steering wheel.