There are hardly any animals endemic to the UK -- found here and nowhere else.
There are about one-and-a-half birds -- the Scottish crossbill, and the red grouse (a variety of willow grouse). Some other sub-types.
There are no mammals, although there are several sub-types, such as the Irish hare and the Skomer vole.
Then there are a few insects -- but hardly any compared with the enormous overall number.
Also very few plants. Lundy cabbage, early gentian, a few whitebeams and eyebrights and one or two others.
It's only a few thousand years since the last ice age. During that there were very few species here, and since then there's been little time for new varieties to develop. Therefore almost all our species have come here recently from mainland Europe, and are still much the same as those that remained.