In theory, unless you're a local driving out of your lane to your house and know what the speed limit is, then going from any side road to another should have a 'new' speed limit sign at the junction.

If not, you should stay at the limit on the previous road, and if you don't know what that was, 30mph to be safe. If I recall, there was (not sure if its still the case, but probably), it used to be that when the street lights were closer than Xm from one another on a stretch of road, then it was a 30mph limit unless signposted (with repeaters every Ym min) otherwise.

It would be nice to know, as I'll be taking my annual jaunt to Cornwall next week for a fortnight and there are many such instances of this sort of thing, mainly on the more rural roads, some of which do have speed cameras and camera vans on.

Both our Audi satnavs have a speed limit indicator, which is really useful, as they can often change after few hundre yards and back again. I don't know how many satnavs have this - the previous Volvo didn't.

But you have to make sure they are regularly updated.

I'd be wary about the accuracy of such satnavs etc for that - my admittedly old-school Here Maps on my Windows 8 Nokia phone (don't ask) has regularly missed speed limit changes, even obviously signed ones that look like they've been there for years.

Admitedly handy when it gets it right as you say (when going through villages on normally national speed limit roads), as mine bongs loudly if I go over the limit (no camera warning system though), even by 1mph (though it is the satnav speed limit, which is about 5% more than my speedo reads, at least at higher speeds).

The 'Bible' that LA work from regarding signing of speed limits -

google

Traffic Signs Manual Chapter 3 Regulatory signs

Go to Section 14 at Page 105

dvd