Before you get out into the fresh air, just to remind your collegues where you are off to, and why you are going, why not use this desk reminder, specially designed by the Work Your Proper Hours Team at the TUC?
Download the sign and print it out, fold the blank side behind the printed side to make it a bit more rigid and then fold in half. Plonk it on your desk one way round whilst you're away at lunch, and simply switch it round at the end of the day to show you're off on time.