World Atlas says that "Loch Ness is believed to contain more fresh water than all the other lakes in Wales and England combined". Notice the use of the word "other".
An article in the Telegraph (dated 21 April 2015) contends that Loch Ness "holds more water than in all of the lakes and rivers of England and Wales combined" (my italics). One on the BBC website agrees, as does Rural Roads.
According to Pocket Mountains, Loch Ness "holds nearly twice as much fresh water as all the lakes of England and Wales combined".
Visit Britain goes even further: "the water volume [of Loch Ness] is almost twice as much as all the rest of lakes in England, Scotland and Wales put together!" This, I feel, may perhaps be stretching credulity a little too far.
© Haydn Thompson 2019