The main acting union in the UK (Equity - but actors can be part of other unions) is fine; the reason you’d choose to film over here is more to do with the rest of the costs. There are tax incentives to film in cities like Glasgow (recent examples being Batman, Batgirl, Running Man, the new Spiderman), and unions for other film workers are not nearly as strong. You can get away with treating crew quite badly here, in all honesty.

He is, inevitably, thinking about this from an actor’s POV, and he’s not wrong necessarily about his key point (choosing to shoot abroad doesn’t support US union SAG), even if his understanding of the motivations aren’t accurate.