Great white egret

I'm interested in how great white egrets are becoming more common nowadays. The RSPB Birds of Britain and Northern Europe (2011) says that their range is southern Europe, Africa and Middle East. I saw one in Oare Marshes, Thames Estuary, Kent last week. Are they rare, or are they becoming increasingly common? Perhaps this is a sign of climate change? Any thoughts as to why they are increasing would be greatly appreciated.

Kind regards