If you like this, you might also like OpenTrees.org:
> OpenTrees.org is the world's largest database of municipal street and park trees, produced by harvesting open data from dozens of different sources.
Cool! I like how the official UK site in the OP avoids having a stuffy generic name and just goes with "Ancient". I guess this is like Java-speak for picking BritishBuild over UKExcludingNITreeFactoryConstructorPattern
opentrees.org seems to have very little data on the UK.
For fans of Giant Redwoods in the UK there is also https://www.redwoodworld.co.uk/locations.htm