gistfile1.c

=if( // #IsPounds:
  iferror( // #FindPoundSign
    find("£",TO_TEXT(E2), // This will error out if it isn't there.
    1) // #IsPounds:True (returned by Find)
    ,0 // #IsPounds:No (returned by iferror)
  ), // End iferror#FindPoundSign
  // This is the result if #IsPounds:True
  E2, // return it in the cell. #GAMEOVER
  // if #IsPounds:False:
    ( // #GetCurrencyConversion:
       iferror( // the GoogleFinance/Index will error out if the day wasn't a trading day #TradeDataExists:
          // Asked for historic information (Date is D2) GoogleFinance will return a 2x2 array, 
          //   which we only want one cell of, Index(range,X,Y) returns that as the correct multiplier.
          Index(GoogleFinance("CURRENCY:USDGBP", ,D2),2,2), // #TradeDataExists:Sucess Set to historic data (Did not error)
          GoogleFinance("CURRENCY:USDGBP") // #TradeDataExists:Failed Set to today's data
        ) // End ifError#TradeDataExists
    ) // End #GetCurrencyConversion
    * E2 // result of #GetCurrencyConversion * Cost in Dollars returned. #GAMEOVER
) // end if#isPounds

=if(iferror(find("£",TO_TEXT(E2), 1),0), E2, iferror(Index(GoogleFinance("CURRENCY:USDGBP", ,D2),2,2) * E2,GoogleFinance("CURRENCY:USDGBP")))