Just pack the equipment in its original box, apply the return label (provided) and take it to a Post Office, click HERE to find your nearest Post Office.
You MUST return the equipment using Royal Mail Special Delivery, from 1st April 2010 the return postage will cost you:
Tomtom One: £5.50
Navman S30, Garmin, Tomtom 710, 910, 930T & Traffic: £6.70
Tomtom Rider v2: £8.65