The article is correct in that the CAA have published their opinion that the Thames is too busy to be considered suitable for ditching and should be considered a built-up area.
Since that discussion on EuroGA (which basically says who cares if you break the law if you don't get caught) mode-S and FlightRadar24 have ensured that you will get noticed.
Edit to add that the Lee Valley lakes are also considered to be dubious for ditching if that features in your plans.