United Kingdom

The Scottish Crossbill (eBird) is a bird that occurs only in the United Kingdom.

The Orkney Vole (ARKive WM), the Skomer Vole (BBC), and the Saint Kilda Field Mouse (Wikipedia) are endemic mammal subspecies.

Freshwater fish unique to the United Kingdom include Malloch's Char (Ittiofauna WM), the Loch Rannoch Char (Ittiofauna WM), the Loch Killin Char (Ittiofauna WM) (BHL), the Torgoch or Welsh Char (Ittiofauna WM), the Shetland Char (Ittiofauna WM), the Orkney Char (Ittiofauna WM), the Golden Char (Ittiofauna WM), the Large-mouthed Char (Ittiofauna WM), Haweswater's Char (Ittiofauna WM), and the Windermere Char (Wikipedia). Endemic whitefish include the Vendace (WCRT), the Powan (Ittiofauna WM) (FRT), the Schelly (CEH) (FishBase), and the Gwyniad (BBC) (Ittiofauna WM).

Beetles found only in the UK include the Bronze Lundy Cabbage Flea Beetle Psylliodes luridipennis (ARKive WM), a scarab Psammoporus insularis (UK Beetles WM) (IUCN), the rove beetles Thinobius newberyi (ARKive WM) and Meotica anglica (ARKive WM), and a weevil Protapion ryei (ARKive WM). Other endemic insects include the Northern February Red Stonefly Brachyptera putata (Buglife pdf) (flickr), the Orange-striped Stonefly Perlodes mortoni (iNaturalist), Fonseca's Seed Fly Botanophila fonsecai (Discover Wildlife), an anthomyzid fly Reliquantha variipes (AEMNP pdf), a gnat Creagdhubhia mallochorum (BHL), a chalcid wasp Omphale erugata (ZooKeys), the Caledonian Planthopper Cixius caledonicus (IUCN) (BHL), a jumping bristletail Dilta chateri (iNaturalist), and the Manx Shearwater Flea Ceratophyllus fionnus (British Fleas) (ResearchGate). Endemic moths sometimes considered full species include the Northern Footman Eilema sericea (UKMoths), the Dorset Tineid Moth Eudarcia richardsoni (UKMoths), and the Slender Scotch Burnet Zygaena loti scotica (UKMoths) (NTS).

Other invertebrates exclusive to the United Kingdom include the British Cave Shrimp Niphargus glenniei (HCRS), the Horrid Ground-weaver Nothophantes horridus (iNaturalist) (BBC), the Celtic Woodlouse Metatrichoniscoides celticus (BMIG) (YouTube), a millipede Anthogona britannica (BMIG WM pdf), a centipede Nothogeophilus turki (IUCN), a terrestrial planarian Microplana edwardsi (GBIF), the sponges Hymeraphia elongata (Marine Species WM) (ResearchGate) and Antho granditoxa (Habitas) (ResearchGate) and Cliona caledoniae (Univ. Amsterdam pdf), and the possibly extinct Ivell's Sea Anemone Edwardsia ivelli (ARKive WM) (Lancing Nature).

Vascular plants restricted to the UK include the Lundy Cabbage (ARKive WM), the Scottish Primrose (ukwildflowers.com), the Cheddar Whitebeam (IUCN), Stirton's Whitebeam (Nat'l Museum Wales), Plot's Elm (flickr) (Wikipedia), the Early Gentian (UK Southwest) (iNaturalist), the Purbeck Sea-lavendar (flickr), the Welsh Ragwort (BSBI WM), the Great Orme Berry (GTC WM) (iNaturalist), the Mountain Scurvygrass (ARKive WM), the Shetland Mouse-ear (Plantlife WM), a monkshood Aconitum anglicum (POWO), the Snowdonia Hawkweed (ARKive WM), a monkeyflower Erythranthe peregrina (Wikipedia), the Western Ramping-fumitory (iNaturalist), the lady’s-mantles Alchemilla minima (ARKive WM) and Alchemilla sciura (ResearchGate), the Cornish Eyebright (ukwildflowers.com), the Interrupted Brome (Wikipedia), the Scottish Small-reed (p. 182 of BSBI pdf), the Gransden Goldilocks Buttercup (flickr), a bramble Rubus halsteadensis (iNaturalist), and the Saint Kilda Dandelion (flickr).

Endemic non-vascular plants include the Derbyshire Feather-moss Thamnobryum angustifolium (ARKive WM), the Scottish Thread-moss Pohlia scotica (RBGE pdf), Dixon's Thread-moss Bryum dixonii (RBGE), the Welsh Bristle-moss Orthotrichum cambrense (MNHN), the Scottish Beard-moss Bryoerythrophyllum caledonicum (Plantlife WM pdf), the Northern Prongwort Herbertus borealis (IUCN), the Greater Copperwort Cephaloziella nicholsonii (ARKive WM), and the Scentless Crestwort Lophocolea brookwoodiana (twitter WM). Endemic lichens include Fissurina (or Graphis) alboscripta (Fungi of Great Britain & Ireland) and Petractis nodispora (Dorset Nature). Endemic fungi include Anthostomella alchemillae (Fungi of Great Britain & Ireland), Cortinarius britannicus (Wikipedia), Cuphophyllus lepidopus (IUCN), and Geastrum britannicum (Norfolk Biodiversity).

For a list of endemic species see (Wikipedia). An overview of endemic invertebrates is at (Buglife pdf), A 2020 listing of endemic vascular plants is at (Semantic Scholar pdf).

See also Anguilla, Bermuda, British Virgin Islands, Cayman Islands, Falkland Islands, Montserrat, Pitcairn Islands, Saint Helena, South Georgia, and Turks and Caicos Islands.