I think it's just that British television is a lot less stable than American television. Seasons are much shorter (6-episode run compared to 22), and renewals seem to be a lot more dicey. For example, an American series generally knows if it's going to be renewed or not before the current season ends.
So because employment in a Brit TV series is a lot less predictable, I think there's a lot more turnover in casts, as British actors are constantly looking for another job. That means that the writers are forced to kill off characters because the actors have left. That in turn makes them more willing to kill off characters.
In American shows, in contrast, it's a lot easier to keep the actors under contract for multiple seasons. And the actors are happy to have steady predictable work, so they're less likely to leave. It's also a lot more work to replace them. So killing regular characters off becomes a rare tool, deployed for major effect.
So because employment in a Brit TV series is a lot less predictable, I think there's a lot more turnover in casts, as British actors are constantly looking for another job. That means that the writers are forced to kill off characters because the actors have left. That in turn makes them more willing to kill off characters.
In American shows, in contrast, it's a lot easier to keep the actors under contract for multiple seasons. And the actors are happy to have steady predictable work, so they're less likely to leave. It's also a lot more work to replace them. So killing regular characters off becomes a rare tool, deployed for major effect.