Glad I found this it's been irritating me all day.
From a marketing point of view these protestors and total bloody idiots.
#BlackLivesMatter is an absolutely essential cause to stop the racist murders in the USA they are diluting and harming that cause!
The issues simply doesn't exist here, not only do not many people get shot and/or killed in any way by police each year, the ones that do are overwhelmingly white 
If they are showing solidarity for the USA, why fuck up British people's days and cost the UK money? We aren't doing it and most of those people probably agree. How is that going to accomplish change? Is the UK government going to say "oh dear a protest at Heathrow lets go and make the USA change its racist stance" nope!
If that's the issue then blockade American companies, start a boycott of American Airlines, March on the US embassy, do something constructive.
If on the other hand this is about poor life chances for black children, then start your own campaign, no one gets this, you can't just jump an issue and go "yes but it's not really about police shooting people, or death at all really, it's about socioeconomic factors that are affected by race, but you know we like the #".., worst strap line ever.