In the very same way that a cheaper price or a similar look does not mean equal quality. There is a connection between price and quality if you go below a certain price level but there is not necessarily one on higher price levels. Still there are a number of factors apart from build quality that explain a higher price of a British made Brompton.

Black Ti fork from 2018 looks like CHPT3. The issue shown in the picture looks like the root cause would have been an overtightened stem nut. The question would be who overtightened it. Brompton is not free from failures but if they occur they usually handle them very generously (like the bottom bracket one lately). And if I remember correctly something was changed regarding the ti-forks not too long ago. So my questions would be:

1. was the stem nut overtightened by the factory or did someone fiddle around with it later?
2. did you place a warranty claim with Brompton and what was their reaction and handling of the issue?

One of the reasons why Bromptons are more expensive is that they actually have warranty, spare parts and customer support...