I too saw the 180 just east of the junction of Oxford and Regent Street.
Have towed alot myself and tried to buy a business some time ago. The rules were you could tow over London/large city but had to use a twin-and meet all the usual regs in the event.........
As for hight( rule 5), not sure, but that was a big banner and having had engine problems towing before, you don't go far and dropping the banner invites all sorts of issues.
It was a long time ago for me, but I can't see that rules have been relaxed. Interested to know.