A list of suggestions so far (with a couple of my own added at the end)
www.english-heritage.org.uk/education/free-entry/free-school-bus-scheme/ - Free bus and entry for English Heritage
Also a list of other places which may be of interest.....
The Epping Forest field centre (exit 26 off M25) has excellent programs for school children and they are free! They can even lend welly boots to kids who need them, and they will need them:-)
The William Morris Museum, in Walthamstow, has been done up and looks great. It's free too with a large park for picnicking
The Gunpowder Mills at Waltham Abbey.
RSPB bird sanctuary in Rye House
Forty Hall
Lee Valley Park Farms
Braintree Museum is a good value day out at £5 per pupil. They do a Victorian day, a WWII Evacuees day & a natural world day
Rainham Marshes
Hadleigh Castle rare breed farm
Hylands Park
Victorian school in Hitchin.
Discovery Centre Stockwood Park Luton
Hendon Airplane museum
Whipsnade zoo
Willows farm
St Albans museum
Welwyn Roman Baths
Roald Dahl Museum
Tring Natural History Museum (or the Dead Zoo in our house!!)
The Paper Mill in Hemel www.thepapertrail.org.uk
Hertwood Forest
Dunstable Downs Visitors centre