More here:
https://vintagetek.org/tek-guernsey-2/ and here
https://www.youtube.com/watch?v=T3u5_a1GElU (Pity the video quality is poor) - there is some reference to tax considerations in the commentary. And this article
https://vintagetek.org/guernsey/ states that Tektronix closed the Guernsey plant in 1990, moving production to the Netherlands.
Other web articles can be obtained.
PMM