There are only a handful of trees that are classed as 'native' trees of the UK. It might surprise you to know that a lot of parks and green spaces will have less native species than non-native. The benefit on native trees such as English oak would greatly outweigh those of a non-native tree such as Norway maple. But how do we define what is native or non-native? 
Based on historical records - around 8000 years ago (6,100BC) Britain and Europe were connected by a land bridge called Doggerland. Britain was essentially an ice sheet with very minimal plant life. As the ice age came to an end the sea levels increased which flooded the Doggerland land bridge and formed the Britain that we know today. Any plant species that colonised the land back then are what we class as a native species. Based on this understanding - there are around 40+ truly native trees.
Native species such as English oak have grown with Britain for thousands of years. This is why they are so beneficial. They know the landscape, the ground and the air much better than a newly introduced species. This is why a native tree such as English oak offers such a huge amount of biodiversity. Typically in the landscape throughout Britain most of the trees and shrubs will be native. It is quite a different story in our parks, public green spaces and private gardens.
Whilst English oak, Common Beech and Common ash are the three most common native trees across Britain it is unlikely you would see all three in the same park. Similar trees such as Turkey oak, Copper beech and Narrow leaved ash are widely planted and although they might look the same from a distance, they are non-native. The same applies for our evergreen trees. There are only five native species of evergreen in Britain which are Box, Holly, Juniper, Scots pine and Common Yew.
Now that we have a better understanding of what all of this means why are there more non-native trees than native? Two main influences are the Romans and the Victorians. When the Romans invaded Britain and began to setup shop they brought over a lot of their plants. This includes grape vines, Black mulberry and Sweet chestnut to name a few. During the Victorian period even more plants were introduced into Britain such as the Monkey puzzle tree, Giant redwood and Rhododendrons. At the time there was a huge demand for rare and exotic trees for private collections.
It's worth noting that quite a few non native trees are now naturalised in Britain and most people will treat these as native. Sycamore and Horse chestnut are great examples of this. The important thing is that we need to be mindful of our native trees and ensure that we are planting them in higher numbers than non-native. This is easier said than done as each council has it's own way of doing things which can lead to problems further down the line.