Printed

A Field Guide to Trees of Britain and Northern Europe by Alan Mitchell (HarperCollins 1976)

Collins Tree Guide by Owen Johnson and David More (Collins 2006)

Field Guide to The Trees of Britain and Europe by Alan Birkett (New Holland Publishers 2017)

Websites

Google Lens: Go to google.com and there is a camera icon at the right of the search bar. Click on this, and you can drag and drop any image and it will do its best to identify it – so can be used for any flora and fauna, vertebrates and invertebrates etc. (as well as buildings, landscapes – anything, in fact.)  Worth a look, but treat identifications with caution!

Apps

Tree Guide UK (app) Standard Edition and Premium Edition (Apple)