I doubt Croydon currently has enough glamour to attract the super rich to park their money with empty mansions.
I imagine that the main reason for property to be empty in Croydon is that it can't be sold/rented at the target price level.
But Croydon actually has some of the most expensive areas of the UK. When people think of Croydon, they often think only of Croydon town centre, but that's a tiny part of the borough. It doesn't have the 10+ million flats you might find a handful of in Kensington etc., but it does have lots of multi-million properties, in numbers big enough that those are significant for these stats.
E.g. here's an example of the type of things you might find in Purley, which is part of Croydon [1], [2]. In Shirley (also Croydon) you're more likely to find properties around the million mark, but you do have roads where the average is around 1.5-2.5 million.
In terms of empty properties, there's minimal difference between those areas, or the luxury flats near the station, or the middle class areas, or the couple of run down council estates.
[1] http://www.zoopla.co.uk/for-sale/details/41807624?search_ide...
[2] http://www.zoopla.co.uk/for-sale/details/33270402?search_ide...
The proportion of empty properties does not vary much across London as far as I know. Certainly not enough to be noticeable.