Giant sequoias: There are less than 80,000 of these redwoods left in California,

[speak easy]

10. Umm ... no. This is a study of Sequoiadendron giganteum in the UK

Giant sequoia (Sequoiadendron giganteum) in the UK: carbon storage potential and growth rates
https://royalsocietypublishing.org/doi/10.1098/rsos.230603.

Throughout the article the author is clear he is studying the Giant sequoias first introduced in 1853 by Scottish grain merchant and keen amateur collector Patrick Matthew "More recently, there has been a resurgence in planting giant sequoias in the UK, particularly in urban settings. This is because of their carbon storage potential and perhaps because people seem to really like them."

Yes, the half a million number conflates "Giant sequoias (Sequoiadendron giganteum) are some of the UK’s largest trees, despite only being introduced in the mid-nineteenth century. There are an estimated half a million giant sequoias and closely related coastal redwoods (Sequoia sempervirens) in the UK. "

But the TFA its about giant sequoias which are plentiful in Britain.