The national currency in Mexico is the Peso. Many places will however accept the United States Dollar, the Canadian Dollar, or the Euro.
You should plan to get any Pesos you want either before you arrive in Mexico to get a much better rate than if you change your sterling up when there. You will (typically) also receive better value for your money if you use the local currency.
To get the latest rates and compare whats on offer, check Holiday Currency Exchange Peso rate page.