The principle of people from A, migrating to B, to seek work has been in place ever since the industrial revolution drew people away from farm labour to "better" pay and conditions, in the city.
The poster could just as well have been of a coachload of people from Newcastle, on their way to London, to take jobs off Londoners, to make his point but UKIP wanted the narrative to focus on migration from overseas. The photo was easy to obtain (because of scale and persistence of the movements) and it *just so happened* that they were a mix of Afghan/Turkic/Arab-looking people, with not a single white face. (And, if they'd photoshopped a few in, they'd have been found out and humiliated for it).
Race is incidental; parking a large chunk of one country inside another country but leaving all the schools, hospitals, shops, offices, industry, water works, farmland, roads and rails, a thousand-plus miles away places the recipient country under stress. The poster just said it should just stop. I think it would have been far more effective to implore the British government to 'ing well BUILD SOMETHING, QUICK!! Carve another blob of grey out of our green and pleasant land. Who knows, it might boost the economy?