Lazy journalism pandering the the DT xenophobic/racist readership.
There is an implicit assumption that all 4000 British passport holders were dual nationals, with an underlying assumption that they were all primarily first generation Sudanese without any evidence being offered to support this view The second assumption, why does someone having the passport of a second country absolve the UK of its responsibilities when asked for help and assistance? Otherwise what is the point of the time and expense of getting dual citizenship if it is of no value? It is not granted on a whim. Finally, there is no mention of where any of these 4000 individuals were primarily resident. Many were visiting family to celebrate a major religious festival.
Could it have been foreseen and should they have travelled? Well, given absence of the ambassador, his 2 i/c and sundry others from the country al at the same time, one concludes either it was not or that they do not read their own advice.