The International Dendrology Society
An international organisation with c. 1500 members in 50 countries.
Currently, the bulk of the text is derived from two sources. The 'Bean' text is from the Eighth Edition, the last to be published, which appeared in four volumes during the 1970s, along with the Supplement, published in 1988. New Trees was published in 2009 and sought to cover all the new introductions to cultivation from the period from the Eighth Edition to c. 2005.
https://www.treesandshrubsonline.org/
The National Fruit Collection
The National Fruit Collection is one of the largest fruit collections in the world and includes around 3,500 named Apple, Pear, Plum, Cherry, Vine and Cob Nut cultivars. The collection is owned by the Department for Environment, Food and Rural Affairs (Defra). The University of Reading is responsible for the curation and maintenance of the National Fruit Collection in collaboration with the Fruit Advisory Services Team (FAST)
https://www.nationalfruitcollection.org.uk/search.php
Westminster City Council map of all their trees
London street trees you even get a google street view of the tree
https://apps.london.gov.uk/street-trees/?_gl=1*auqp2h*_ga*MTU2NzE1NDQ2My4xNjg3NTM1OTc5
i - Naturalist
I created a Wirral Trees set of observations within i-Naturalist.
https://www.inaturalist.org/observations/wirraltrees