If you are in Writer, your chances are slim. Writer doesn’t have powerful formulas for calculating. If you are working in Calc, which makes more sense, you can of course use a simple formula to do the conversion - if you convert dollars to pounds, multiply the values in dollars by the exchange rate. If you have the values in the C column, you’d get something like
=C2*0.73
Of course, that will only work if you didn’t include the dollar signs in the cell content but in the formatting. If you have C2 = “$1234.56”, formatted as text, you first have to delete the dollar signs, then format the cells as currency before you can do the math.