A1/M1 Barnet Bypass

There is a sign on the gantry above the start of the A1 / M1 that I cant find the meaning of.

Its here , although not in this picture.

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Where the 30 currently is , it looks like the National speed limit sign , but has a red bounding circle.

Does anyone know what it is or have they just been lazy and put in in the orginal 30 sign and forget about the red circle ?

i think they are just too lazy to put a new national sign up.
there is a national sign further along as well.

i would point out, that that has been waiting to become a national for a while, 30 was just far too slow.