It's a price-drop by Amazon, to match the lowest retail price of the day, which is currently ASDA. These price drops are not always for promotional reasons. For example, some months back, Amazon dropped the Wii U 32GB version to £150 and 8GB version to £100 for one week. It was to match ASDA's price drop when ASDA was getting rid of their stock.
Here, it's ASDA who must be pushing PS4 sales. Amazon are just matching the lowest current price.







