There will always be signs at the start of a change in speed limit, and, if more than 30mph, there will be repeater signs. The problem is with trusting satnavs which haven't been updated. There's also the problem of temporary speed limits for road works. These are often not clearly marked, and IME often don't have the end of the new speed limit marked. And usually a satnav database won't have these updated as they are only temporary. Come to think of it, are they updated for motorways which may have months or years of roadworks?