There was sea upon sea of white faces.
If this had been a BBC drama or an advert of some kind then the field would have been at least 60% non white. But this was real life.
If the population of Britain is (according to the stats) 10% non white then there should have been about 10% non white runners in the race, but I'd say there were about 1% non white (max).
I myself really don't know.