What is the speed limit?

This post is relevant only to UK residents.

When I first learned to drive in the 80s, there was a rule that if the street lights in a 'national speed limit' zone are more than x yards apart, the limit was 60mph, but if the spacing was less than x the limit was 40mph. i always thought this was daft because I would never be able to judge that distance.

Fortunately, soon after i passed my test, they revised the highway code and removed that rule.

That left it as:

Urban areas 30mph
Single carriageways 60mph
Dual carraigeways 70mph.

There is no longer a 40mph National Speed Limit.

Yeehaa....zooooooom.....laugh mockingly at fuddy-duddies crawling along at 40mph every time they see a street light.

BUT

I looked at the highway code today, and it says in the small print for the asterisk note that if there are street lights it is 30mph unless signs show otherwise.

"* The 30 mph limit applies to all traffic on all roads in England and Wales (only Class C and unclassified roads in Scotland) with street lighting unless signs show otherwise)"

This means that those fuddy-duddies are [shock horror] SPEEDING! And they thought they were such good drivers!

But lets sit back and think about this for a minute. Many motorways have street lights. There are only National Speed Limits signs, nothing showing a different limit, so does this mean that you have to do 30mph on the motorway? likewise on a dual carriageway, does it become 30mph at the first hint of a street light? I don't think so. So on a normal road, say an A road, does this rule apply? And if not there, does it apply to the more countryfied roads that I am thinking of? What I understand this rule to mean seems to be totally impractical and in fact dangerous if people do suddenly slam the brakes on to meet the 'limit'.

So - I am very confused. Can anybody clarify this for me please?