I tend to agree (Alec Guinness being another great example), though I think it is also about how big a deal live Shakespeare was in Britain in the eras those actors came up. A lot of good British actors were in Hamlet because they were talented and of age.

It'd be extra convincing if there were more non-Brits with Shakespeare backgrounds that became big Sci Fi actors to point to. There are American Shakespeare companies or performances, but it's a much smaller deal.

ETA: another source of great sci-fi actors was Hammer Films, which produced Christopher Lee and Peter Cushing. Great American sci-fi actors come from more backgrounds because there's more acting jobs in America.