Well, in the last couple of years pounds to dollars translations have gone in the following way: just add $50 or $40 to the number of pounds, and you have the dollar price (not exactly rocket science). Now with the lower value of the pound, there might be a change in that way of doing things, especially considering the Switch might have a low starting price (rumoured 199 pounds by Laura Kate Dale). The Daily Mirror guy said he heard 299 pounds and 349 pounds, but his source is from retail, and he himself said that the source doesn't have access to the direct information (so, in theory this guy could simply be reporting the place holder price for the Switch).
Take a look at
this video (mark 50:12) to see how the PS4 Slim and PS4 Pro did the conversion of pounds, dollars and euros.