I'm sure that having your broadcast used by protestors to make political points is a little bit annoying, but only if you think that your selection of what counts as information is paramount. I'm inclined to feel that if you go for an outside broadcast, you should accept what's going on. Otherwise, you may as well stay in the studio. Few things are more patronising than insisting on showing you the door to No. 10 to demonstrate you're talking about politics, or standing outside Buckingham Palace to remind you that that's where the Queen lives.
This is Nick Robinson, the BBC's political correspondent. In case you wondered about his political views, he was President (sounds dangerously Republican) of the Oxford University Conservative Association, and therefore isn't in any way biased.
I wonder why he didn't politely complain to the sign holder, or simply walk away.