Thank you Iain for this insightful opinion.
Indeed, FTDNA does not seem to have completely succeeded in erasing prediction errors due to bias in its databases.
An illustrative example of these prediction biases, that you are not without knowing, with the branch R-S775 > R-L745 > R-FGC34909 (> R-S781 - downstream of R-P312>>R-L21), illustrates due to the House of Stuart.
The Stuart, as well as the FitzAlan, descend from the Breton knight Alan fitz Flaad (+ ~1120), seneschal of Dol-de-Bretagne. These 2 lineages were established in the United Kingdom in the 12th century. Their most distant known ancestor (MDKA) was Alain, dapifer sacrae ecclesiae Dolensis archiepiscopi Dolensis, alive in the 11th century.
Therefore, upstream of R-L745, Globetrekker should pass through Brittany (France), for at least a century or more (between R-S775 and R-L745, almost 2500 years have passed!), or this is not the case... Globetrekker indicates that R-S552 (a descendant of R-L21) would have crossed the Channel around 2 600 BCE, and the entire lineage from R-S552 to R-S781 would have remained in United Kingdom...
As a certain number of Bretons originally came from Great Britain, there is a significant possibility that this lineage actually came from Great Britain, then migrated to Brittany around the 6th century, before returning to settle on the other side of the Channel, from the 12th century.
Cheers,
Ewenn
Ewenn