Protest signs written in English allows UK and American news cameras to capture the anger in a way that appeals to news consumers in the UK and US, and even Europeans, most of whom read English.
It instantly transforms what could have seemed a far-off problem - with signs written in Greek, which is literally "Greek" to most Americans and Britons - to something close, easily understandable and "real."
Signs legible to hundreds of millions of people around the world allows the protest to be "branded" as something of international, not just local, interest. And that makes it far more valuable as a news story.
I'll also note that most of the signs look like they are from a single protest. The lettering on on "Hands off Cyprus" sign looks the same in several (meaning: dozens) of wire service photos that have been used for four days in a row now (this coming from an informal Google image search.)
This is even more amazing, since a small number of signs have been effectively transformed into the LOOK of this story for days now. This shows the power of the idea of non-English-speaking protesters appealing to English-language readers.
I often wonder why protesters in Middle Eastern nations like Egypt and even in the Palestinian West Bank don't use more signage in English, and adopt protest techniques proven to be successful in Western nations. My advice to them is to watch the Cyprus experience and learn from it.