Here is a list of tree species found in the UK:
Alder – Common, General, Grey
Alder Buckthorn
Ash
Aspen
Beech
Birch – Downy, Silver
Blackthorn
Black Locust/False Acacia/Robinia
Box
Bracken
Cherries – Bird, Wild/Gean
Chestnuts – Horse, Sweet
Crab Apple
Dog Rose
Elder
Elm - Wych
Eucalyptus
False Acacia/Robinia/Black Locust
Firs
Gorse
Guelder Rose
Hawthorn
Hazel
Holly
Hornbeam
Juniper
Laburnum
Larch – European, Japanese
Lime – Large Leaved, Small Leaved
Maple – Field, Japanese, Norway, Silver
Oak – English, Holm, Sessile, Turkey
Pine – Corsican, Scots
Poplar – Aspen, Black, Lombardy, White
Robinia/Black Locust/False Acacia
Rowan
Sitka Spruce
Sorbus
Strawberry Tree
Sycamore
Tulip
Western Hemlock
Whitebeam
Wild Service Tree
Willows – Bay, Crack, Goat, Weeping, White
Yew
Also posted in 'Open Pastures & Woodlands' and 'Flowers & Plants'.