There us actually an embryonic emerging science to “political science.” It has a lot to do with understanding public opinion and how it can be harnassed.

In “legal” terms attack on foreign soil is an attack on foreign soil.

That said, the poltical reality is different.
You can reasono expect different British public opinion results in each of the following cases:

  • Russia hires a lifelong British criminal, a Liverpool street thug, enlisting him as hitman, who then stealhfully kills Sergei Skripal and his daughter with close range bullets to the head.
  • Russia “sends a message,” killing them in public with a nerve agent, but in a close-up personal manner, with zero chance of errant collateral casualties.
  • Russia sends a drone and bombs Sergei Skripal’s 8-story apartment building.
  • Russia carpet bombs London.