That really depends on the exchange rate you get when you exchange your sterling for pesos. Not used my Nationwide card in the Philippines yet, but when I've used it in the US/Europe the exchange rate given was basically the current mid-market rate. So unless you know of a money exchanger that gives the mid-market rate I can't see cash being any better (and could be a lot worse).
The best way (though obviously not always practical) is to not use cash and pay instead with the the credit card issued by the Post Office.