The blue circle with a red cross in it is the sign for no stopping (usually meant for clearways). It’s very hard to find a visual symbol to represent ‘No Stopping’, however somehow a blue circle with a red cross is the most logical choice.
The colour depends on the road and the purpose, while blue signs are for motorways, green signs are for normal roads, brown signs are for tourist information, and yellow signs are for diversions.
‘No Stopping’ sign means that drivers must not stop in the area shown by the arrows on the sign, even if they are just picking up or dropping off passengers. Likewise, drivers must not stop along sections of road marked with a continuous yellow edge line.
Of interest, UK regulators are reportedly moving forward with a plan to ensure that Google Glass users don't wear their headsets while driving, well before the controversial eyewear even reaches Blighty's shores. Find out more at The Register.