You can find all the native British trees listed here, and we grow and sell most of them. This list includes all the trees & big shrubs that are considered native.
The definition of native trees:
"True natives trees" are often defined as:
Trees that colonised Britain during the time between the end of the ice age about 10,000 years ago and the formation of the Channel by the gradual expansion of ancient rivers, some thousands of years later.
Trees that came after the Channel had formed are generally called Naturalised.

List of Native British Trees & Shrubs

Plant GroupNative Trees in the Group"True" Native Tree?
Alder Alnus glutinosa Yes
Ash Fraxinus excelsior Yes
Aspen Populus tremula Yes
Beech Fagus sylvatica No (Probably brought by stone age humans)
Birch Downy Birch, Betula pubescens & Silver Birch, Betula pendula Yes
Blackthorn Prunus spinosa Yes
Box Buxus sempervirens Yes, South GB only
Buckthorn

Alder Buckthorn, Rhamnus frangula - Purging Buckthorn, Rhamnus cathartica & Sea-buckthorn, Hippophae rhamnoides

Yes
Cherry Bird Cherry, Prunus padus & Wild Cherry, Prunus avium Yes
Crab Apple Malus sylvestris Yes
Dogwood Cornus sanguinea Yes
Elder Sambucus nigra Yes
Elm English Elm, Ulmus procera - Wych Elm, Ulmus glabra - Smooth-leaf Elm, Ulmus minor Yes for Wych Elm. The others are disputed & only in the South.
Hawthorn Hawthorn, Crataegus monogyna - Midland Hawthorn, Crataegus leavigata Yes
Hazel Corylus avellana Yes
Holly Ilex aquifolium Yes

Hornbeam

Carpinus betulus Yes, Mostly South GB
Juniper Juniperus communis Yes
Lime Large Leaved Lime, Tilia platyphyllos & Small Leaved Lime, Tilia cordata

Yes, South GB only.

Maple Acer campestre Yes, Mostly South GB.
Oak Common Oak, Quercus robur & Sessile Oak, Quercus petraea Yes
Poplar Black Poplar, Populus nigra & Aspen Poplar, Populus tremula Yes
Rowan a.k.a Mountain Ash Rowan / Mountain Ash - Sorbus aucuparia Yes
Scots Pine Pinus sylvestris Yes
Spindle Euonymus europaeus Yes
Strawberry Tree Arbutus unedo Yes, Ireland only.
Whitebeam Sorbus aria Yes
Wild Service Tree Sorbus torminalis Yes
Willow Goat Willow, Salix caprea - White Willow, Salix alba - Crack Willow, Salix fragilis - Salix triandra - Salix pentandra Yes
Yew Taxus baccata Yes

Although Britain has many native trees, there are effectively no endemic trees, i.e. trees that are native here and nowhere else. If all the trees in Britain were destroyed tomorrow, the world would not lose a single distinct species of tree. The small exception is a handful of asexual micro-species of Whitebeam, Sorbus aria. These apomictic plants are a bit of cheat if they want to be held up as unique: they naturally occur in Whitebeam populations around the world and each strain only counts as a distinct species because they reproduce by cloning only. They are in all other respects the same as normal Whitebeam, so they are not of any special importance to an ecosystem.

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Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut et massa mi. Aliquam in hendrerit urna. Pellentesque sit amet sapien fringilla, mattis ligula consectetur, ultrices mauris.