I've just been told there are public showers in Gatwick South - a well kept secret.
Look at this map:
http://www.gatwickairport.com/globalass ... arch16.pdfOnce you clear customs and exit through the World Duty Free shop (#12 on page 5) turn hard left and take the stairs or lift marked "Lifts and stairs to local buses." You want to go up one level to Departures (Level 3) this will take to the area behind Joe's Coffee House (#7 on page 6), the showers are between the toilets as you exit the stairs/lift. You will need to use the courtesy phone to call for the attendant who will bring the key to open the shower, soap and a towel for you to use. The whole service, including the use of the towel is free.