Separate ticket opened about sanitychecking these kind of issues in general -> #42940
About the only sane thing to do is to set nationset to "all" in the scenario. There's no point in having the start positions defined for the nations if they are never used. We could remove the nation definitions from the start positions, but the idea of the British isles scenario is to have british isles with local nations, isn't it?
The problem with this is that supplied, official, scenarios shouldn't rely on "lower quality" (i.e. more likely to be missing) translations of the legends and nation names in the nations translation domain. That was the point of splitting translation domains and introducing whole nationset feature in the first place.
My proposal is: Set nationset "all" in the scenario, but because of the slight controversy related to translation domains, do not target this to mature S2_6, but only to S3_0 and later.
Patch attached. Set target to beta3, but we'll see if that holds.
Reported by Molo_Parko in http://forum.freeciv.org/f/viewtopic.php?f=13&t=91988#p104022 . Of the nations that have startpos in british isles scenario, only English show up (other startpos get filled with random other nations). This is because those other nations are not in the "core" nationset, and the scenario uses it (as it's the default).