I burn them they're a pain (I'm an Electrician).
They all have "returnable refundable" printed on them, but nobody pays a deposit on them. Generally the bigger, solid wood, ones are returned to the cable suppliers. The smaller (plywood) ones are reused by electrical wholesalers, if they are clean.
Find your local electrical wholesaler (City Electrical Factors, Senate, Newey & Eyre, Edmundson Electrical, Denmans) and go and have a chat - avoid the busy time of first thing in the morning - they'll probably give you them for free (they have to pay to dispose of the dirty ones)