Thornberry Tweet?

I'll confess my absolute ignorance here, and maybe one of yez Brits can enlighten me on this, but what is the deal with Emily Thornberry posting a picture of a white van? It was a big story last week on all the British news outlets, everyone seems to agree that this was a terrible thing to have done (she even lost her job over it), but not a single article that I can find discusses the meaning of it. It's apparently something which is glaringly obvious if you live in the UK, but I've not a clue about it. I was expecting some US news outlet to pick up on the story and explain the meaning of it, but none of them have said a word about it, quite possibly because it's baffling to them as well. What's the deal here?