I mean, that's probably part of it too, but I also feel like they wouldn't have filmed the scene at Canary Wharf.

Honestly, given that Robin Hood's home was Sherwood Forest, Sycamore Gap is about...checks Google Maps...about 172 miles by (modern day) road out of the way, too.

EDIT: Hmm. Apparently, Acer pseudoplatanus also didn't grow in Britain at the time Robin Hood was supposed to be running around, either -- it was introduced from central Europe, probably significantly later:

https://gabrielhemery.com/native-trees-of-britain/

Some trees introduced a long time ago to Britain are now considered ‘naturalised’. There is a specific term for species present since 1500; an ‘archaeophyte’. Such species include beech (native only to south-eastern Britain), horse chestnut, sweet chestnut, sycamore and walnut.

https://www.wildlifetrusts.org/wildlife-explorer/trees-and-shrubs/sycamore

It was introduced into the UK from Europe sometime in the 15th or 16th century, and has become naturalised since, as well as being widely planted.