Loch Ness

... is only Scotland's second largest loch by area (after Loch Lomond), but its great depth means that it holds far more water.

Wikipedia contends that it "contains more fresh water than all the lakes in England and Wales combined", citing the Gazeteer of Scotland (which doesn't actually mention the word "fresh" in this context; but I think we can probably agree that all of Loch Ness's water is fresh).

A Bing search provides more on the same theme. For example: World Atlas says that "Loch Ness is believed to contain more fresh water than all the other lakes in Wales and England combined". An article in the Telegraph (dated 21 April 2015) agrees that Loch Ness "holds more water than in all of the lakes and rivers of England and Wales combined". One on the BBC website agrees, as does Rural Roads.

Notice that nobody I've quoted so far uses the phrase "twice as much". So I changed my search to include the words "twice as much water". This revealed that Pocket Mountains agrees: Loch Ness "holds nearly twice as much fresh water as all the lakes of England and Wales combined". Visit Britain goes one step 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 last claim, I feel, may be stretching credulity a little too far. But the consensus does seem to be that Loch Ness contains more water than all of the rivers and lakes in England and Wales, and nearly twice as much as the lakes. The claim that it contains "nearly twice as much fresh water as all the rivers and lakes in England and Wales combined" would seem, however, to be a conflation of these two comparisons.