Beds are sometimes listed on the Freecycle website:
http://uk.freecycle.org/
The British Heart Foundation have quite a few charity shops which are dedicated to the sale of furniture and electrical goods. You might find something suitable there. Enter your postcode and select 'BHF Furniture and Electrical shops' to find your nearest store:
http://www.multimap.com/clients/places.cgi?cli ent=heart
You might also find a suitable bed advertised in your local 'Free Ads' paper.
Chris