(but remember on allotment plots you can only plant fruit trees, not any old species)
free trees for woodland
trees for wet soils:
https://www.rhs.org.uk/advice/profile?pid=152
Atlantic White Cedar, Bald Cypress, Black Ash, Freeman Maple, Green Ash, Nuttal Oak ,Pear, Pin Oak, Planetree, Pond Cypress ,Pumpkin Ash, Red Maple, River Birch, Swamp Cottonwood, Swamp Tupelo, Sweetbay, Magnolia, Water Tupelo, Willow -
https://www.gardeningknowhow.com/ornamental/trees/tgen/wet-soil-trees.htm
willow for weaving - http://www.yorkshirewillow-shop.co.uk/willow-cuttings/
native trees frn Stanmer Park
http://www.specialbranchtrees.org.uk
Brogdale for fruit trees, root stock and advice
http://brogdaleonline.co.uk/rootstock/