Bossk and an A-Wing rebel flight suit first appeared in this vintage Doctor Who episode.
This is from Doctor Who episode The Tenth Planet (1966). One can only guess how this happened. I would hypothesize the Lucasfilm production scoured England’s prop masters for things resembling flight suites they could “edit.” Star Wars Insider once wrote that the flight suit Bossk where is actually designed to be really cold and was an actual space suite of some kind. So that doesn’t necessarily mean the suits used in Doctor Who are the same ones used on in the Star Wars films. However, it would seem if some space suits made their way into British television programs, it would be more likely the Star Wars productions would have acquired those.
Apparently Bossk shops at the same stores as these space men from Doctor Who.
The A-Wing pilots must have thought Bossk was so cool they too would shop there.
You can watch the episode here:
The Tenth Planet Part1 by matrixarchive
Edit: Forum post and regular Now, This is Podcasting! contributor @WC_Animal dug up another image for us and lets us know that The Tenth Planet was the first regernation by the Doctor as well. This is from The Wheel in Space: