Democracy is a tricky thing. If you believe in it, you want to believe in everyone's right to free speech, but does that include freedom of action? And where does that freedom end? Or does it? For instance, if that action has the intention to harm then should that freedom be curtailed or restrained or be subject to legal action?
When the right to protest impacts longer term democracy should you be morally or legally constrained? For instance, since the Pla(in) Stupid protesters clambered on to the roof of the House of Commons, access to the home of Parliament has become slower, more restricted and far more time consuming. It used to be a particularly good thing that as a member of the general public you could at any time amble up to the Central Lobby for a look around. It's openess gave you a sense of being at the heart of something important, and the fact that you didn't have to petition for access meant it felt public-orientated.
With the security clampdown we have lost a little bit of that.
The custard protest - by the same group - could have the same long-term effect. Basically, it raises questions about the high-level security around members of the Cabinet as they walk to meetings, which may chase them into only travelling in secure cars surrounded by security operatives at all times. In other words, it pushes them away from the regular world, where you can just run along a road to a meeting, or get public transport, and away from the world that the rest of us operate in.
An immediate response from a nearby onlooker when watching the custard thrower was: "that would never happen in the US, they would have police surround them in a second".
Most of us would rather we didn't operate in a world like that, and think it is better for politicians not to either.
Plain democracy means we want our institutions and politicians to be as open access to all as possible, and not sealed within a security bubble.