On the motorway there are signs above the road, electronic boards, they have orange speed signs:

'Congestion Slow Down'

Then they have a speed limit sign, saying 60 or 50 or 40. They are not the Variable speed limit signs like on the M25.

Are these orange speed limit signs a point of law, are they enforceable?

They are advisory signs only. Not legally enforceable AIUI.

What does AIUI mean?

As I Understand It!!

Thanks, I honestly had no idea what you meant.

Forum abbreviations...

Unless its with a red circle around it, its not enforceable, merely advisory (although exceeding it COULD be classed as driving without due care and attention), even with a red circle it MAY not be legal, but wouldn't suggest gambling with it!

Simon

IIRC (<---Should keep the OP busy for a while!)

The Lane closed (Big red X) is enforceable, and you commit the offence as you pass the sign.

Could be wrong tho'

Not wrong at all. There was a case recently where a HATO (sorry OP!) observed a car ignoring a red X and a prosecution was made.

Highways Agency Traffic Officer, I know that one!

Have always wondered about this .... so why are national speed limit signs enforceable i.e. no red circle???

The black & white 'de-restrction' sign was in service before the NSL. Since the NSL applied to every road without a lower speed limit it was adopted as indicating NSL.
This avoids changing thousands of signs every time the Govt varies the NSL for the road type. Also diff vehicles now have different NSL limits for class of vehicle so 1 sign fits all.