Sort of. That used to be the case, and it's still what they teach on those courses. However there are now "20mph zones". These are signed on entry, but do not have the repeated small 20 signs of a normal 20mph limit. This means that you can no longer tell whether you are in a 30 or a 20. I have once seen something marked as a "40mph zone" but I suspect that this was a local aberration and did not have a similar rule.